Uturn, I am no vet, but it seems to me it is boundary time. If you have them, then you should start enforcing them and if you do not have them, then it is time to get some in place. If she is to return to you and the marriage IMO there can be NO FURTHER CONTACT of any sort with the OM. No friendship, do friendliness, no NOTHING. If she waffles on this then I would consider this a deal breaker.
So in a nutshell, I would do the boundary thing and she can decide what next. IMO you CANNOT nice her back and if you allow her to have contact with OM, you will just get screwed over (literally).
