awest, I would suggest NOT to tell him you know about the convo. I see no reason for that. Everything I ever read says that there is a "gap" bewteen deciding to break up with the other person and actually doing it. It happened to me as well (to my H).
So, keep things light and easy. Now YOU have become the OW as far as she is concerned. Only difference is, their relationship is more fragile and she will probably push him further away.
