Sparkelpop, thank you for your insightful and compassionate words, and SchrodingersCat, I like the way you cut to the chase.
I realize that people need to paint a picture based on my short summary in order to frame their advice, and while I acknowledge that I may also be biased, I would like to assure you that M is not remorseless or heartless. She doesn't trash talk her husband and struggles with the ethics of our continued communication as much as I do. But we do work together and have to communicate professionally anyway, and it just seems absurd not to ask how things are going now and then.
I can't speak to the details of her negotiations with her husband, but I do know that's she's concerned with his emotional stability and is probably making whatever concessions she feels she can live with to help him through this. When she did share that we had even one conversation it was devastating to him, which is why she's probably chosen not to pursue it further for now. Not ideal, I know, but I'm not going to judge her too harshly.
Some of you mentioned that I just focus on making new friends, and I certainly agree, but please remember that S and I are still very new to this as well. We're trying to take things slowly and she isn't quite comfortable with the idea of new friends yet. Also, S has expressed that she would be OK with me resuming a relationship with M if it didn't work out with her husband. I suppose it's easier for her to accept a known quantity.
38 M, Straight, Minneapolis. “Love is that condition in which the happiness of another person is essential to your own.” -Robert A. Heinlein, Stranger in a Strange Land