If you are anywhere near campus, a nice bike is gonna remain yours for about a week so my vote is he's trying to ugly it up a bit to make it less of a theft target.

Since he's a young guy, he probably dropped a lot of his savings on it so he's protecting it from bike rack damage.

Since people are screwing with your bikes, I wouldn't leave the saddle bag and bike computer on it if I were him, but maybe I'm paranoid. Also, he doesn't have the cable going through his rims. He's in for a rude awakening.

Admit it, you checked his odometer didn't you?