Caz, re your comment about contacting her, to be honest i would do (and did) exactly the same. There was no benefit in it for me but the things to think about are firstly, why should the little creep get off scott free. And secondly, no matter how much we try to deny it, revenge IS sweet!
The thing to emphasise if you are going to do this, is that you have to be quite honest in order to seem sain. I started with
"dear PERSON, I don't know if you know who I am but I am NAME's ex, who I have just dumped having seen his recent correspondences with you. I read his email, which was wrong of my but I had my suspicions which, it turns outwere absolutely spot on. So, I wanted to tell you that he has told me he cheated on you blah blah etc etch"
And ended with "I know you have absolutely no reason to trust what I am telling you and, as you can probably guess, I am telling you this to get back at him because he has been an ar*ehole to me but please remember that I have no intention of going anywhere near the lying sh*t again, so have no interest in trying to split you up (if indeed, you are actually together) - I just wanted to make sure you were aware of, what I am told to be, the facts of the situation and could make as informed a choice as possible".
In my case, she didn't listen to me - he cheated on her 6 months later. Then she left him (and wrote the same emal to the person he was cheating with in her case). And she's now a very good mate of mine.
So it all worked out well in the end!