The first thing you do is take a few deep breathes. Calm yourself down before you do anything. Anything you decide right now will go horribly wrong.
I know this must hurt a great deal. But, any reaction to him directly will crater all of the hard work you've done.
Look at this way, he is moving. What kind of relationship can they have now. We men sometimes need a little excitement in our lives. Someone who makes us feel special (trust me here). She must be doing that for him. Try to do so as well. I know its hard because of the trust factor, but it can work.
