(Good call, Ex! It is indeed not a PeVi(Me) spell - by the spell description, it's a PeVi that affects only Mentem Spells, not the Form of Mentem. If possible, something affecting Mentem magic would be more appropriate, rather than a "mental/psychological flaw" or effect on the apprentices "mind" - but with a roll like that, such "technicalities" can be ignored.)
Okay, so the apprentice has MuMe effects on him, and the PC hammers a PeVi spell to an notorious degree.
Chaotic Magic reads "...If you... exceed (the declared level of effect of a spont spell) by more than one level, the spell still works, but its effects are beyond your control - the storyguide decides the results..."
Which makes tracking those spells harder for the Quaesitor - good call for improving the story!
KB - first - was the apprentice an NPC's, or a PC's? (The Quaesitores???) And the apprentice themself is an NPC, right? If all NPC's, you can be much harsher without risking ruining the game for anyone.
Hrmmm... PeVi that is "beyond his control"...
It could harm the Gift of the apprentice, giving him a weak Art in Mentem. That might not be enough for a march, but certainly earn the PC an enemy (two if you count the Parens), and the real threat from the apprentice won't show up for some time (long after the PC has forgotten him?) A Flaw like Hatred or Driven...
It might be a ~permanent~ PeVi(Me) effect on the apprentice - no Mentem spell will work on them (or no MuMe) ever again. This is mostly a good thing, tho' the Parens may not think so. (And it could make further investigation more difficult, and possibly create a real Hermetic headache for later Quaesitores!)
The apprentice might now have a magical Side Effect, of PeVi(Me) or something, destroying Mentem magic. This won't be evident for some time, until the apprentice casts a large enough spell near a Mentem spell effect.
(Or destroying any memories, whenever he casts a spell. Maybe not permanent destruction - just for a day or so?)
His own Mentem spells might be extremely vulnerable to dispelling (meh - bad for him, fairly minor in the overall story, unless he's a PC.)
Mmm - without knowing exactly who this apprentice is in the group/story, it's hard to get a feel for any balance - you want something significant - if an NPC, the effect would have to be much larger - if the apprentice of a PC, it could be more subtle.