First off, you probably already know this, but erasing the memories of a hermetic magus is a high crime and one that is likely to result in a march.
Erasing memories is not in itself very difficult. The problem is doing so while making sure that you only erase the bits that you want to erase.
You would want some spell that erases specified memories (I would require a Finesse roll to ensure that a) the spell only deletes what the player wants, and b) the spell actually deletes all of what the player wants, with many botch dice).
The player probably also needs a spell to view the memories of another person, in order to sort through the memories of the Tytalus to make sure that they find all of the problematic memories.
Of course to even begin to pull this off the player also needs a means to subdue but not kill the Tytalus. This in and off itself is IMO likely to be the more difficult part. Considering that the Tytalus is extremely likely to be prepared for some version of this scenario to unfold. Your player also needs to be able to subdue the Tytalus without anyone figuring what is going on. This leaves them in a dilemma.
If they use spells to do it, they leave traces for a quaesitor or allies of the Tytalus to find. If they dont use spells then they have to defeat and subdue a hostile magus quickly without magic.
They need to bring the Tytalus to a location where they can start the memory deletion, again either risking a magical trail or having to figure things out without magic.
They have to complete the memory deletion fast enough the no-one notices that the Tytalus is gone and they have to find a way to insert the Tytalus back in their natural habitat in such a way that the Tytalus or their allies will not become suspicious about it and start to investigate.
This is no mean feat.
If the Tytalus mage has any smarts about them they will have written down what they know in some amount of detail, and made more than one copy of the text. They will have placed this text somewhere where they are likely to stumble upon it every now and again. Likely at least one copy in their sanctum, probably at least one other copy somewhere else, that is hidden and not easy to link to their person.
This way someone who erases the problematic memories will need to have extensive insight into the mind of the Tytalus magus.
If the Tytalus has their memories erased they will then at some point in the near future find the text describing what they used to know, put two-and-two together and realize that their mind was attacked and take action accordingly (which may be nothing, but probably wont be).
It is also possible that the Tytalus has told someone else the secret or done something else to help them in case the blackmailed mage tries to retaliate. This could include things like making sure that incriminating evidence will be found upon their disappearance or death. Depending on their level of paranoia this could also include leaving a trail of breadcrumbs and deleting their own memory of it, so that they will rediscover the trail only after having their memories deleted. The potential for mind games is limitless.
The only limiting factor on the potential for preparations is resources. leaving a description around is both very resource efficient, easy to implement and effective.
All things considered, to be honest, the memory erasing plan is pretty stupid. I am not saying dont go for it, but in my opinion it is extremely unlikely to end well for your PC. There are just too many things that can go wrong.
In my opinion your player only has two options that are merely bad but not stupidly bad. Those two are:
Go big: If the player has the means to subdue the Tytalus, abscond with them and secure enough time to erase their memory, then your player, by definition also has the means to just kill the Tytalus, which would accomplish all the same things. True in this case people will start to wonder why the Tytalus died, or why they are gone. But then the challenge is "only" to make the death seem accidental. That in itself is no small feat, but IMO it is a lot easier than the plan proposed by your PC.
or
Go home: If your player decides that the risk is not worth it, or that they are unable to go through with the plan, they can either try to accommodate the Tytalus. Considering the nature of house Tytalus there is a good chance that the Tytalus is not really planning to go to court over this, but merely trying to prove that they can best your PC magus at this game of "I pwned you so hard you have no choice but to beg for mercy". Conceding defeat may even be enough to make the Tytalus pull a Rick Sanchez and say "Thats it, now I am bored with this game". It might also be possible to make a bargain with the Tytalus to make them back off, ideally a bargain that is somehow enforced either magically or legally, to guarantee mutually assured destruction (maybe your player can even find something equally naughty on the Tytalus). For example: Your player could try to find something enticing but extremely illegal to pay off the Tytalus, because this way the Tytalus has committed a high crime herself by accepting this highly illegal payment.