Would You Tattle on a Cheater If Your Friend Was the One Cheating?

In recent years, I've determined that there are times when it's appropriate to stick my nose into someone else's relationship and times to keep mum. Knowing when to do it is the difficult part. So say, for argument's sake, I had a really good friend who was cheating on his serious girlfriend. Do I say something or not? (P.S. I need your help).

Here's the situation:

I have a friend who has been with his girlfriend for two years. He's always going on about how he's going to marry her someday and blah, blah, blah. The "blah, blah, blah" is because while he's gushing about her being the best thing ever, he's also been cheating on her. It's never with the same girl, and sometimes it's only hardcore flirting, but the whole thing makes me really uncomfortable.

The thing is, I'm not close to his girlfriend at all. My allegiances are with him, and his friendship is important to me. Is it worth sitting him down and telling him how uncomfortable I am or that I'm considering telling her? If I do, I assume he'll tell me he's not cheating anymore and then make sure I never find out in the future. Or do I go behind his back and tell his girl? That seems deceitful, and I don't think I could live with myself. I guess I could also just let the relationship play out, but then I feel like I'm in on it and guilty by association.

What do you guys think? You all had such amazing advice about prenups (just last night my best friend Madhu said how impressed she was with your comments), I was hoping to turn the tables this time and be the one to ask for advice.