First take James' advice and find out the real story instead of listening to rumors.

If it's true and he is cheating here's my advice:
IMO, once a cheater, always a cheater. There are too many good guys out there for you to worry about that loser for one more second. Get rid of him, forget about revenge, and don't look back. Karma's a bitch and he'll get his eventually.

So true.;)