I am sorry. I hate to be a downer to all the lovely suggestions here, but I would walk. There is absolutely nothing more important in a relationship than trust. PERIOD. Once that is gone, there is nothing left. A bit dramatic? I personally don't think so. I agree with Miner... definitely not too dramatic to walk.

Now... would I give him a 2nd chance? Nope. Why? Because it would never be the same. Maybe that's just my jaded-ness from watching too many failed 2nd chances in relationships, but once that deepest level of trust is broken once, there will always be the suspicion that it will happen again. She will always wonder in the back of her mind what he's doing... and she'll always be looking over her shoulder--she deserves better than that!

It's crap. On top of which, I have to say this... the "Dom" (and I use that term LOOSELY here) is crap for treating her like that. I am not saying he's a horrible person in general... ok, yes I am. That's HORRID. Be a man. Own your behavior. Don't be a douche & go behind your girl's back, and then try to smooth it all over by acting like it was part of your "act" in the first place by trying to somehow "tie it in" to BDSM. It's cheating, plain & simple... dressed up or plain, it's still cheating.

Just my 2 cents.