The rear wheel locked on him while ridingusually if it is clutch material it catches in the pockets of the molding on the cover etc. like it was held there by the centrifugal force. I think I would take the clutch apart, look for the loose nut, see if you have some wear, and that takes a 15 dollar set of calipers (something every one needs to measure "everything") and the wear specs. if he used old springs you get all sort of problems if the old ones ended up sacking out at different rates, so measure them and make sure they are the same lengths.
When you say locked, what exactly locked?