Heheh. NSA, yes, though I already work for the
DoD. Be afraid...be VERY afraid!

Seriously, though, trying to reset other peoples' passwords is, in my mind, just one step down from outright hacking. AKA it won't be tolerated either. That's like walking through someone's neighborhood and checking all the doors to see if they're open, and I don't think people would say THAT was anything less than an invasion of privacy, and at worst, a crime.
Just make sure he knows that ain't gonna fly in the future. He's welcome back, and I mean that. But he's going to have a shorter leash for a while.