Pain meds or not. The vet obvious gave the meds to take home, for a reason. The cat probably didn't take the surgery well, really painful for the cat, so he opted for the client to take meds home. Also, we do not know, if the meds are causing the d'rea. Are we really in the right notion, to tell someone to stop taking anything. no, I don't think so.
She really should be consulting her vet and figuring out why her kitten is having d'rea.