Secretly record the pig talking about his other girl or get them on video. Show this to your ex and then leave it in her hands. If she insists on dating him despite the evidence, they deserve each other.
Walk away and go find yourself someone with more self-respect :)
If you want to tell her, you certainly can; but don't expect her to stop dating him because of anything you say. Most likely, she'll want to find out for herself; so you might be better off by leaving it alone and getting on with your own life. The choice is yours.
why be afraid youll seem like a jealous ex, you are, arent you? if you have real intentions of trying to get back together then you should be honest with her about that. if she doesnt have any more interest in you then its none of your business who she dates but then you can still tell her what you know without fearing it will be a cause to change her feelings about you.
If you know he's cheating on her tell her. You would want to know wouldn't you? But by 'know' I mean you should have compelling evidence. Suspicions aren't enough. You should also bear in mind that if you're wrong your ex will probably never forgive you for it and you'll probably lose any chance you might have had to get back with her.
Unless something prompts her to actually ask about the situation, stay out of it.
I'd stay out of that one She will find out if she doesn't know already
Leave them be
First of all don't wait around for someone who is trying to see where it goes with another guy before possibly coming back to you. Go find someone to whom you will be first choice.
If you still feel the need to help your ex by revealing the cheating, get some facts to give to her and not just your own evidence.
Sounds like you're not over her!