How should I act toward this girl that stole my best friend's boyfriend?

[Sfsun]

My friends Daniel and Kelly just broke up. They are both very dear to me, and I knew a break-up was coming, but Daniel had to go and cheat on her before they broke up. That was only two weeks ago, and now he's dating the girl he cheated with (who knew Kelly and had invited her to hang out before - this is all messed up!)

So I'm still friends with Daniel, but have been very clear about where I stand on his actions. I have been counseling Kelly - she's stayed at my house the last two weeks and is just blown apart by all this.

The problem is, my boyfriend (also friends with Daniel) got some tickets to a play tonight. So I'm going with my bf, Daniel, and his new girlfriend. I haven't met her yet.

I'm nervous. I know Kelly can never find out, it will break her heart. I'm still friends with Daniel, and if this girl makes him happy, I wish them the best. I just wish they had done it differently. I'm not going to be rude to her, but I don't know quite how to act.
Honestly, from what I know about this girl, she seems perfect for Daniel, and if he's going to date her then it's inevitable that we'll be hanging out eventually. I'm sure she's a nice person, but they conciously chose to cheat and they both knew what they were doing. They weren't drunk or anything.

I'm thinking about taking her to the side and telling her that I think what she did is really messed up, then just leaving it in the past and trying to get to know her as a person. Everyone makes mistakes, so I'm not going to hold it against her forever. But I also feel like I'm betraying Kelly...