How to get a Facebook user to change their password?

Question

Recently I found a printout with a facebook user's username and password. I logged in as them to see if it was legit, and lo and behold, it was. So how can I tell this guy that he seriously needs to change his password without revealing who I am, since I wouldn't believe my story if someone else told it to me? I'd rather not get in trouble for doing the right thing here.

Answer 1

Could you log in on his account, change something or put in a line saying "Hey Dude! I think you need to change your password," so that he will hopefully notice that someone has logged in?