Second gear slipping, what could it be?

A bike a friend of mine is possibly interested in buying apparently has some sort of transmission problem. The seller says that it slips only when it's in second gear, all other gears are fine. I assume that because it only happens in one gear, and no others, it rules out a clutch problem. Any ideas what else it could be, and am I correct in assuming the case would likely need to be opened to fix it? If so, he'll probably pass on the bike.

Thanks!