Create a lower level spell not sure how much lower but depends on your capacity for spell mastery and rebuttal bumps the affected spell level by 3*mastery score. Master it like you need for Fast Cast. Master it further for Rebuttal, Penetration, Adaptive Casting (if possible), Quick Casting, Precise Casting, Stalwart Casting. Also, take a snip of their hair and fix it as a permanent AC.
Oh, yeah, and Disguised Casting (if possible) so that if he’s harmed or dies the Quaesitors have little to go on when they realize that yes it was their spell but it was altered with MuVi.
You know, or you could just talk to them IC or, if that doesn’t work, OOCly about playstyles. And you should definitely make sure they’re cool with PvP unless that’s already been discussed.