I really wish you hadn't said that- because this is exactly how I feel about her. I'm trying not to, for his sake, but yeah.
As for her marriage, what I hear is that he knew from the beginning that she had been poly, and this made him feel less than secure about being able to satisfy her, both emotionally and sexually. She agreed to be mono, but apparently edited her marriage vows to not include anything about being exclusive.
Then, after the kid was born, she suddenly needed other people in her life, and without negotiation, she found some people online and started "dating" them. It was about this time that he found out she was a serious sub, and wanted/needed a dom- he had no inkling of this before, and was not, as far as I could tell, at all dominant. He tried, but apparently, she ended up topping from the bottom, which only made him feel more self-conscious.
She's the one that made the ultimate decision that the marriage was over- after she'd moved away (she had a job here, he didn't, and he was going to finish packing their stuff).
From what I've seen of her relationship with Sanguinist, she demands attention a lot- he even asked me at one point if I could "be the sane one" for a while, because he couldn't handle any more drama than what he was getting from her. Not that needing attention is bad, but he's always exhausted after he talks to her- even when she's had a good day.
I should mention that he's aware of my concerns, and tells me he does not intend to have her move in unless there's a plan for these things to work out. I trust him to stick to that, I'm just really nervous that something will come up to destroy that plan.
As for avoiding me, he says that she's had bad experiences with trying to have female friends in general, and with trying to be friends with people she's sharing lovers with in particular. I respect that she has fears, but we've met several times- and if she doesn't like me, can't she just say so?
All of this becomes more troubling for me when I remember that Sanguinist's ultimate plan is to have a big poly house where we all live as one big family. How can we do that if we can't all communicate now?