You told her that OM didn't even try to fight for her when you confronted him. She may need to see for herself that he doesn't really want her and that he's just in it for what he can get.
You and OM's W have applied pressure and it just caused your W to want to contact him more. Do you happen to know which one initiated the contact?
By her pleading and saying she needs help, may indicate that she now realize she's really in a pickle. I think it was good for her to see you standing firm and bringing into light how she canno have both you & OM, and that there cannot be any progress as long as she continues to have ant communication with him. An A has to end before anything else! It was very good that you did not let her get by referring to him as a friend. Hold her feet to the fire. The more she can get by through telling lies, the more she will do behind your back with OM. Call it by the right words and don't try to shine it up.
It may take a breakup before things turn around, but do not agree to just separating, like she suggested, b/c she would see it as you giving in. She would simply continue in the A. She needs to believe you will D her if she doesn't end the A. Let her wonder what you are thinking and what you may do.
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!