That was the wrong thing to do...throwing away his check. And you probably knew it was a check because you opened his mail...another bad idea. You should have just written "return to sender" and put it back in the mailbox.
But you're interested in the future, not the past. The easiest thing would be for your ex to just contact the sender, ask them to cancel the first check and issue a new one with the appropriate address.
You haven't committed identity theft, but you could technically be charged with tampering with the mail or destruction of personal property. I think most police agencies would be hesitant to actually charge you with anything if you didn't try to cash the check. But that's up to them.
Try the replacement check route and see how it goes. Good luck.
I'm not your attorney. By offering "answers" on Avvo, I am doing nothing more than providing an opinion for informational or educational purposes only.