I lent my bike to a friend today. On bringing it home, he reported that while in 1st gear, the hub had suddenly become very rough and grinds in 1st and 3rd gear. 2nd gear is working as normal, but in 1st and 3rd the hub is stiff and 'notchy', leading me to think that it's a problem with the internal gearing.
I've oiled the hub, and as yet it's still rough. Any ideas as to what's causing the problem?