Tell her if she were truly remorseful, she'd not keep in contact with the other guy in the first place.
When people make a mistake they generally do not want evidence of it hanging around. The fact that she stays in touch with him makes me think she doesn't really see it as a mistake and is telling you what she thinks you want to hear. Even though she didn't technically cheat, alot of people would have huge problems with the fact that she got with someone else so soon after your breakup. It makes the whole thing with him seemed planned.
Let it play out. You need to know that she's just as committed to working on the relationship as you are and right now she's doing a shitty job of proving that.