Most likely culprit is either a malware infection or a problem with his system's USB root hub(s).
The first is relatively easy, if tedious, to fix. You'd need to run multiple scans using more than one antimalware utility to be reasonably assured you got his machine clean. Better would be to do a clean OS reinstall if he's seriously infected. (Tell him to be a little more careful where's he's visiting - and a lot more careful where he's getting his files from next time.)
A USB root hub issue is a little harder to deal with. You'd need to clean out all
the USB mass storage drivers with something like GhostBuster
or a similar utility. Then add the devices back one by one and let them reinstall themselves.
If it really is a USB issue, that should fix it..
Right now, I'd be more inclined to put money on malware causing the problem for his machine. Especially in the wake of some of what you've told us about it.