If it is a friend or neighbor then it may be border line gossip, and possibly cause more harm.
So, are you saying that you would not have believed OM's W if your step-son had not told you what he saw?
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She's not over OM yet. If OM gives her half a chance (and maybe even if he doesn't) I think she'll continue the A and just get more sneaky. I don't say this to hurt you more, but to hopefully keep you on your toes. I can tell you that when my H confronted me that I was no where ready to give up OM or to work on the M.
I think your W was too far gone when she was busted...to stop the A that easily/quickly...without trying to do it again and not get caught the second time around.
It is not about what you feel should work in your M. It is about doing the work that gets the right results. Do what works!