One player said to me that:
Bonisagius may go to anyone's library and ask to copy a book, just as they must allow anyone form copying their own books - in the spirit of sharing knowledge in the order. I just do not find it anywhere.
Please help me
Nope. They can steal your apprentice if they justify that it is for the betterment of the Order. On the book thing they are obligued to share their research, but they cannot steal your research at all. So it is a one direction thing: he is forced to share his research, but you are not forced to share your research with him.
IIRC there are entries on this in HoH:TL and the Durenmar entry of the Rhine tribunal. In the later book it states that house bonisagus considers it enough that you send your research conclusions to the great library for you counting to have fulfilled your Code obligations. other magi might not agree, though.
It's rather a stretch to say that the Oath/Code allows a Bonisagus can go into anyone's library and can copy the book. Of course, he should seek to clarify this at Tribunal, or Grand Tribunal. 
It is a stretch. But it also sounds like a cool story seed for a legal/tribunal plot. Perhapes in Latin a Quid Pro Quo interpratation is more defensible. My latin is way to rusty to know if that's even possible. Or it could be in the peripheral code somehow.
It would certainly require a lot of politicking on the part of the PC, to ensure that his view is enshrined within some Tribunal's Peripheral Code. I think it's a worthy goal to work for. The journey is probably much more interesting and valuable than the end result of being able to copy any book.
Oh, keep in mind this virtually destroys Cow and Calf, with respect to Bonisagus magi. Unintended consequences and/or motivated adversaries and all that.
Covenant of MegaAwesomeBooks has several well regarded tomes that they've acquired over the years, but none of the copies they have were penned by residents of the covenant. If such a right were granted to magi of Bonisagus, he could walk up to the gates of the covenant, and demand access to the library and copy the book, bypassing the rights of the author[1].
[1]Covenant's uses the phrase seller's permission, where I take it to mean author's permission.
Cow and calf is about seller's permision. The author might be opposed to his work being copied, but unless he put a clause in the contract he has no say on this. Cow and calf is about from whom you got your copy regardless of the authors wishes. At least that is how I have always read it 
It is one of the rules in covenants that I like.
It's much easier to presume the seller equals the author, or should be written as the author, or his assigns. Author is even more compelling when the phrase "originating covenant, or magus" is used further down the page under the Excellent Books section.
There is some room for Hermetic Law to be made here, too. It's generally accepted that the property of a magus is inherited by his hermetic descendants. Do these rights also transfer?
That brings up another tribunal worthy challenge. Using the requirement that Bonisagus knowledge must shared to subvert cow calf restrictions.
That might be a good target for a Tytalus apprentice turned Bonisagus late in his apprenticeship.
Sending the text to Durenmar has generally been viewed as sufficient. Any magus can go there and read the book, for a fee. Or come to him and read it, for a fee.
That is incorrect. In fact, it's not even a stretch of what's correct.
The Oath of Bonisagus magi is different from that of magi from other houses. Bonisagus swear to share their knowledge. Other magi swear to surrender their apprentices to Bonisagus magi, should they ask.
Well, when I was going with rather of stretch, it was an understatement... 
Sure but how many non-Boni Magi could be convinced that's not enouph. Particularly in Saga's where Durenmar is played stingy or worse.
It's all relative, sure.
I'm not sure I'd want to play a Bonisagus in that saga...
If your players want to stir trouble, go right ahead. That's the fun of Ars Magica.
Isn't one the fun parts of political plots getting the players on both sides of an issue.
While everyone is largely in agreement that no, the Code does not grant this power, I'll just play devil's advocate for a moment to suggest that, even if it did, it wouldn't be as serious as it sounds.
For one thing, the book would have to be available. In other words, if a magus of the covenant was using the book, Generic Bonisagus would not be able to access it. This would fall under "deprivation of magical power." It might even be on loan, which happens a lot in the Order. So yes, the covenant has to own the book, but it also has to be accessible.
Second, much of a book's Quality is, as detailed in Covenants, actually a result of scribing, illumination, and binding by professionals with Ability 6 or more. The best books are all clarified, a magical process. My point is, sure, Generic Bonisagus can copy the book, but it's going to lose up to 5 points of Quality in the process.
The covenant could insist on Cow and Calf, meaning that Generic Bonisagus could not, for practical purposes, recopy the book. If he did -- for example, to make a high Quality, clarified version -- it would by rights belong to the original covenant.
And, finally, if a covenant doesn't want to make the Generic Bonisagus comfortable for the seasons he is copying, they can really make his life miserable. They don't have to supply him with a sanctum, or even a room and a bed, if they don't choose to. He'll lose all ability to cast spells inside the Aegis.
A wise Bonisagus would approach this issue with respect and courtesy. He would offer his hosts vis, obey Cow and Calf, and bring Grogs and servants who could help the covenant out during the season in which he is copying. He would be a good guest, and not expect books which the magi were using. Phrased in that way, his chances of being welcomes and granted access to the library are much higher, Code or no Code.
I am happy to bow to superior knowledge, but that doesn't sound right to me. My reading of the Code is that the Bonisagus merely needs to claim that it would be valuable to her in her studies. What am I missing?
Nothing You just worded it better. I only worded in in a way that is acceptable to me. Otherwise IMO Bonisagus would be a very small house due to mass assassination of them (magical or otherwise) if this was not the case. You placing a tractatus in Durenmar would certainly not satisfy me if you abducted my child.
Well I play 3rd edition, so Bonisagus is a very small House - it hadn't occurred to me that was the reason though.