As long as he is using motorcycle specific "wet clutch" motor oil, then my money is on worn dogs on the #2 gear. Split the case and replace the gear, or better yet, have all the transmission gears undercut. If they are too worn then one or more gears may need replaced, but I highly recommend undercut gears if you split the cases.
Worn or bent shift fork may cause a similar issue. Also requires splitting the cases. Shifting technique can minimize the problem, you just have to isolate the "zone" where RPM and engine load "sweet spot" have a less tendency to slip out of gear. There are also some mods that can be made to the shift linkage that some have reported to minimize the issue as well. I've read up on the linkage mods, and plan to try it eventually, but I can't remember where I found that particular information.
