Well if he succeed in stealing her away from you, was she was worth being with in the first place? At least the silver linning is now you are more sure of her loyalty.

As for your friend--well better to let all those around you know so he may not cause other friends heart ache. But for revenge--there are better reasons for seeking it then over something so small. Your best revenge in this case is to never have anything to do with him--if you mattered that much as a friend to him, he would have respected your relationship