I have to tell you tottally, I'm in a similar situation as yours and I feel your concern as well. Those are my thoughts as well. How the heck can I let this go on with OM while working on myself. In the long run it seems to me that her relationship with him will go stronger while ours gets weaker. But I have to admit, the way I'm seeing it now is that she has these feelings for OM, no matter what I do or say and one way or the other they have to be resolved. I can't make any decisions for her and I can't change what "is". The only option I have is to focus on myself and my relationship with my girls and be here for my wife in any capacity that I can.
"Achieve success, but without vanity; Achieve success, but without aggression; Achieve success, but without gain; Achieve success, but without force." Lao Tzu