well obviously by rolling a stress die and studying from vis!
First of all, the art involved ic Rego. Second, the real big problem for game balance is the penetration this guy have with all his spell (spont and formulaic).
I don't want to limit the XP gained, because the RAW permit this and the player took the risk of using vis in a strong aura.
He is a Bonisagus and I think he will use this new knowledge for his career, despite the risk for his life or freedom.
Vasili
That's not the point, the point is that it is not wildly out of proportion to other methods of raising an Art Score.
As I said, via a summa of Level 30, Quality 20, it takes about 6 years of continuous reading to get to Art Score 30, from 0.
Someone studying from vis in a level 5 aura could do the same in about 11 years (assuming average rolls, they get about 11 XP a season). Someone studying from a teacher with Teach + Com of 6, can do it in about 8 years (they get 15 XP a season).
Also, there are likely a number of lower level Summae with Quality around 20. It seems likely that most archmagi have a Score of around 40 in at least one Art --- which means that they are able be write a Summae of Level of 10 and Quality around 20.
So, yes, a Level 30, Quality 20 summa would be great, and certianly it would be prized by the characters in game. But it's not significant enough to Change The Order.
I don't think this is necessarily a problem. It's always possible for characters to have a high penetration --- it's just easy for this character when he uses a rego effect.
Try to think of his unusual position as a way to create stories.
Personally, I think this is a wonderful opportunity for the troupe to create stories. Maybe the character is hired by others to cast wards, aegis, spells to bind creatures, move covenants (or mountains), etc. There is lots of story potential to be had here in travelling to places, meeting other characters, getting involved in the plots of other magi, or even just refusing to help because he is busy in the laboratory. Maybe he is suspected of being an infernalist, as he is very good at warding against demons.
The problem is making sure that this character doesn't overshadow the others, but even that is relatively easy. They can have adventures nearer to home while he is away in another tribunal binding some spirit...
You probably need to discuss with all your players where they want to take the game at this point, and whether they're fine with keeping that character in play as a primary character...
Given he's a Bonisagus, you might just have found your next Primus. That, alone, should keep him busy and leave some spotlight time to other characters. Plus the expectation that he produce his Opus Magna (which should keep him busy for a few years). Then he'll be expected to pass as an Archmagus. And all the stuff others have mentioned. He probably hasn't had time to acquire true friends and reliable allies, so once his secret is out, he probably won't have many people looking after him out of sheer benevolence. Even his covenant-mates are potential avenues for others to gain ascendancy over him.
For the Penetration problem, you have the usual option: demons! With supernatural beings, he's got the basic options of "Ward" and "Command". Tempt him into diabolism. It's so much more interesting to make them do what you want rather than just warding things off... and a little bit of Rego goes a long way. If he's sponting command spells, he might not pay attention to the actual Realm of the being he's trying to command, and the demons might... humor him for a while (remember that they just "know" things).
The lure of Goetia, which work nicely with Rego. That is something he doesn't know yet about his Art. His only hope to keep progressing.
Where did the Rego vis he studied come from? What form did it take?
Which version of the rules are you using?
In 5th edition, your lab total does not include a die roll.
However if you decide to experiment, then you add a SIMPLE die roll to the lab total and then you roll a stress die on the extraordinary results table (p.109).
Regarding XP gained from studying... your study total equals the source quality + applicable virtues - applicable flaws. No die roll is included here.
No dice are involved in the gaining/spending of XP.
Studying from vis, page 165. Source quality is stress die + aura bonus. No lab totals involved.
Hmm... somehow I missed that.
Since it requires spending one pawn of vis per level of art, then a simple cap of gaining one level per pawn spent seems appropriate. For example, you have a score of 5 in the art, you spend 5 pawns, therefore you could only gain 5 levels...
Lol guys, why limit the player?
He play the game and jackpot...
why say now "ho, a moment please, we forget to say if you have the jackpot, we only give you 200 $. Go away. Now."
He took the risk, now, he has the power.
BUt, its like the other say. With great power, great responsabilities... and as a bonisagus... hum,we can say a LOT of people will hear of his power.
That's a funny big story.
He is perhaps a boss in Rego, but still, in others things, he is not the best, and as someone says he has still a deadline: his age.
If it was Creo, i wouldn't have imagine the askings of all the archmagi for longevity rituals potions.
With Rego, he would be asked for aegis and others powerfull things. So, i think his next 100 years will be busy^^
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He took the risk, he didn't blow himself up, and he doubled his score in one art? Isn't that enough?
Do you really promote giving this character more power than anyone else in the entire history of the Order of Hermes? Can you provide an example of an official ArM character with an art score higher than say... 50? 60? 70?
Its a bug, an error in the system which requires a fix. I provided a simple fix.
Your fix is false : so, a 1 1 1 1 1 1 1 1 6 is not better than a 1 1 1 1 1 1 1 4
... then why have the fun?
Its not a bug, because a SINGLE art don't make you the god of the magical world.
In the characters of arm, we have a archmagus master of Perdo... 35+3.
But, well=
- we don't have the name and characteristics of all magi
- what about the FOunders
- you think ALL the great magi will let them known by the others?
I dont think its a bug.
For me, 40 is a practical limit, you need so much studying if with "middle" study totals that, if you are generalist, doing study but also learning of spells, items, familiars... you are too old.
It's not like if at level 80 you reach the "ultimate level of spell power"...
That's the wonderfull world of Ars Magica:
even with a 80 +, say, 20, + bonuses of 20 (and i'm quite generous here), you only have a 120 level of spell (and there, we almost forget the penetration).
Interesting, but not breaching game.
And, its not like if this player had 80 in all arts. We don't know.
Maybe he has his 80 in rego, but only 4 in other arts...
And that's interesting, because NOW, his life will be that everybody knowing his power will see him like a rego master, only a rego master, but nothing else.
And that's the player's choice to be:
- used by allies / enemies
- becoming a leader
- renew the Order and his knoweledge...
For me, it's a great time...
And for the others players, it's time to be quiet effective as "best enemies" near the center of the whole attention of the Order, or "best friends" with a magus who will have a lot of troubles,
It's a hook for saga many SG would like to have... because it was "in game", and occurs when nobody can expect it...
And now, the players and the Order have to deal with that.
A "easy" way to come back is to fast kill the magus, and some people will go for it,
another is to ask this magus to write some summae and tractati before he die...
Because NOW, his time is limited, for the best and the worse ^^.
Exar, like this kind of thing.
I'd suggest you take a good look at that die. Do you think the player would be willing to part with it for a good price? 
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For what it's worth, I'm with the people who say let him have the Art at 85, and start thinking about the stories that that creates. They should be stories about the stupendously powerful Art, however, not stories that mean he can't use it.
It does make scenario design a bit trickier, because he can do anything with Rego, but you can write scenarios where using stupidly powerful Rego magic intelligently is the key to success. Still a challenge, but one that lets the player enjoy his luck.
If you aren't already using some kind of troupe-style play, though, now would be a good time to start, so that the other magi have a chance to be out of his shadow.
And I'm with Erik: see if you can get the player to give you that die. 
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I can also see him developing a delusion: To be the chosen one. (What ever that means...)
I would ask the player what he wants and what kind of stories he would like to play in. If he wants to keep all XP - fine run apropriate stories. If he prefers to end his art score at 40 - fine give im something aditionally to compensate the loss, like a high level spell he would be just able to reinvent and run apropriate stories.
I would love to see something like this happening in the saga I am alpha SG... but chances are... well not good... 
Thanks to all!
I have some new good idea, now. We (me and the player) will talk about the future...
The saga will be more interesting from now on!
Vasili
P.S. we already put that dice under a glass bell and we tribute to it some regular sacrifice! May "The Dice" bless you!!!! 
Also remember this, no matter how high the rego, you can still blow up if you botch in a high opposing aura.
Or buy him a dice cup?
Or if it really was blind luck, can you get him to pick me 6 numbers for this weekends lottery
If it was my saga I'd go for a consensus decision
- The character retires
- The player agrees to a lesser result
- You reshape the entire saga around the Master of Rego, the birth of a new House and the rewriting of the limits of magic to accomadate the spectacular discoveries he's going to be making.
The rest of the players can be his acolytes / minor magicians basking in his aura
How your troupe wants to play the game is of prime importance to how you should handle this.
If you give him the lvl 85 in Rego, much of the campaign will be centred around him. You can give him flaws to compensate or not, he will be the centre of the story.
This can be quite fun, but I know some players don't like to 'just tag along with the other player's hero.' If your troupe likes to do so, go for it.
If you simply cap his xp around 200, it feels like robbing him of his 'jackpot.' Most players don't like this, even when they understand it's best for the game.
Pick any option you like. Here's my idea (that I discussed a bit with Thijs, wh ois in my troupe, hence the similarity):
You could give him around 200 xp and a major magical focus in the Rego area (or some other major Hermetic virtue). This would still make him archmage material, but not totally unique.
You already mentioned the twilight experience. Studying from Raw Vis gives you insight in the fabric of magic. Normally this is expressed xp in an Art. This Magus however had such a revelation that his learning cannot be expressed purely in Rego xp. He also gets a Mystical virtue related to Rego. Perhaps he instantly understands a mystery and could now start his own cult.
With this solution, you still create a strong character. You're not robbing the player of his super-die-roll, since he's getting stuff cannot normally be gained from Vis. But you're not creating an extreme in the sense of the rules, so the fabric of the game doesn't have to change too much.
But as I said: it's up to how your troupe wants to play ArM.
The only thing you can be sure of is that no one will pick Rego for Certamen when challenging him.
Forgive me, but I have to ask a question that has been plaguing me for the two days that this post has been alive.
I thought 1,1,1,1,1,1,1,1,1,6 yields a total of 3072. What am I doing wrong?
Anyway, I wouldn't do a thing (other than say "wow" a lot), especially without knowing any other details of the saga. I don't think other characters necessarily have to immediately fall to the wayside, or lurk in Uber Rego's shadow.
In our saga, one of the characters (inadvertently) became the prima of House Mercere. It definitely changed the saga, but her political power didn't throw the rest of the PCs into the dustbin, or close the curtains on the saga. We're just telling different types of stories, but they are just as fun as when the PCs were Redcap nobodies.
Personally, I'd let the game go on unchanged for a few sessions, notice how the new Rego master changes the game, and act accordingly.
Matt Ryan