Yourname's response hit it on the head. His friend must be quite slick.

If it were me, I would bring it up unexpectedly and see how she reacts. Approach the subject as if you were talking about the weather, keep it gentle and relaxed, and start it off by telling her that you will forgive her even if you really won't. In that way, you might be given a better chance at arriving at the truth. Whatever you do, do not show her any anger or a hint of you being upset, it will make her want to cover it up even more! Be stoic when approaching the matter, and deal with the consequences later...     Best of luck to you, I hate to see that happen to anyone.