things: worn out disc, oil leaking onto it, some change in adjustment, what else? ... I'd think oil leaking onto it would be pretty obvious by now. If there's been no changes to clutch adjustment or parts recently, it's not that. If it's the rear main, he has to go in there anyway. ... And I have seen some guys do a lot of clutch slipping just as a bad habit, which would wear it out pretty quickly.