You are doing exactly the right thing by trying to distance yourself and your marriage from this woman. This essay speaks to me of your attempts to do the healthy thing here and your husband's problem in not being able to do so: http://libidablog.com/how-to-avoid-problem-people/2011/
He can't save her, and allowing her to cultivate a false belief that he will be in a position to date her again isn't helping, it's enabling. There's a very important distinction -- does he get the difference? She needs to find her own ways to be strong, not build her emotional strength on a foundation of sand, i.e. misplaced hope.
So, she feels the situation is "hurtful and unfair". Yes, being dumped hurts. And no, life is not fair. But she's not owed a shot at your husband just because she's into him and is willing to claim whatever changes it takes to get back into a relationship with him. Nor just because he feels sympathy and affection for her, which he's allowing her to manipulate in unhealthy ways.
This is all sooo messed up! If I were in your position I'd be feeling insecure in my marriage too inasmuch as I'd be shocked by my husband's series of poor judgement calls -- sleeping with her without talking to you about it first (wtf?), keeping the relationship going even when it became clear she was a cowgirl (what person who prioritized their marriage and had good sense would keep someone close who would love to see it destroyed??), and now refusing to cut ties even though it should be clear at this point that it would be better for ALL involved to do so.
I'm not sure what there is to gain in you talking to her. She's not a reasonable person, so what fruitful outcome could there be?