Not all poly is threesomes or triads with everyone sleeping together.
His blaming you for not trying to get her back for him is, pardon me, bullshit. You shouldn't have to arrange his relationship with her. That is HIS business. I'd be supportive with a shoulder to lean on, but, I would never ever be the go-between or matchmaker.
Also, if I were the lover and were looking at dating one or both of a couple, I would be paying attention to the marriage and how the couple were treating each other. Any guy willing to neglect his wife for me is no man I'd be dating. Once the new relationship energy wears off, that's how he'll treat ME. If he isn't treating her the way I'd want to be treated in her place, I'm so gone....
I'm perfectly willing to support someone who is going through something rough. I'm not willing to shoulder blame for someone else's actions. There's a huge difference there.