Most Helpful Guy
As much as you'd think you'd like to hear it, and perhaps you might have been a person who could handle it, most people don't want to hear that sort of thing. It sounds good on paper, but in reality it doesn't work a lot.
You're the ex. You don't want him back, you're: Trying to break up their happy lives. (even if it isn't true)
1) Now he's got more ammo against "how he broke up with you."
2) You're trying to break them up because you secretly want him back (not true, but how he or she might spin it).
3) You're the vengeful ex girlfriend who is trying to break them up because you want him to hurt (not true? Still might be how she might see it)
4) You're "butting in." (May not be true, but again, might be how he'll spin it or how she'll see it)
As much as you might want to help, you're a stranger to her. And if he's as big a liar as you say, he's already probably made you look really bad. If she believes him (probably the case), no matter how good your intentions might be, they'll be seen badly.
You can tell everyone else, though... don't go out of your way to badmouth him; don't add in extra personal attacks, as it'll make you look petty... but if anyone asks, simply put, you found out he cheated, so you dumped him. It's someone's duty to tell her (a true friend or family member) perhaps... but not yours.
Stay away from him, don't be "buddy buddy" with him, but just privately realize that it is what it is, and you got out before it could have gotten worse. Good luck.