Gah.
I hear ya man. Girls can be more trouble than it's worth sometimes.

That said, if you've moved on; then you've moved on. Tell her that, be up front and don't sugar coat shit. NOthing is worse than having the person you love (this case, she loves you) lie to you just to make you feel better, and then confuse you down the road. Believe me on that. NOTHING is worse than that.

Be honest, tell her up front that you were hurt and have moved on, you'll be there as her friend for as long as she needs you, but not in a relationship. Stick to it.

Remember what you've gone through, and don't feel bad if she has to go through the same thing. Not that it's "fair", but you certainly shouldn't feel bad for a mistake she made.

Now, go out with that other girl and have a good time.