My flatmate's cheating on her boyfriend. Last night she got this guy over and they were cozying up in front of me. I know she has a steady boyfriend who drops by often. I don't want to get caught in this mess.
Well, its not your mess I agree ... but I also feel that if you hold back information from someone, which allows a person to take advantage of them, you are in a way a conspiritor; you are being an "enabler".
I would personally talk to the roommate and suggest she talk to him. If not, I might tip him off in a way to allow him to discover it for himself.
I'd stay out of it, I think. If you confront your roommate, it'll make things awkward at home. If you tell the boyfriend, the roommate is sure to find out and it'll make things awkward at home. It's a baaaad situation to be in, but your home life may be easier if you just hold your tongue.