Just something to point out real quick, when a friend of mine was taken to court for uttering an idle threat back in high school (he was cleared of all charges), the police came to search his stuff. When they decided to take his computer, they didn't take just the hard drive; they took the entire computer. And not just that, the monitor, speakers, keyboard, mouse, and even his webcam!

I asked a friend of mine's dad who is a lawyer about this. It turns out that by just using another monitor, the case can be argued that the computer was tampered with, and thus all data on that computer was void in court.

Pretty messed up stuff to say the least.

If you are sure it is him and you want to press charges, don't do anything to the computer, but act quickly. If you care to let it go, then format and start over. Even if you do decide to take action, after everything is said and done still format and start over.