I also said it wrong, i.e. handle on your own. Yes, we do seek others, but there is a huge difference between friendship and a sexual relationship. No one can ever question my seeking out my SIL, my uncle, and an old school friend who had been through similar things. And I had no sexual relationship until divorce papers were in hand. At least when I started mine, I was up front with the woman.
As for saving the marriage, that is the choice to make. If MMB wants to save her marriage, then stop running around with this guy and get to it. And if she is going to continue the sexual relationship, then she needs to be honest with the object of her carnal desires.