In the mean time, he's decided to go to therapy. I hope that it helps. I do not want to see their marriage break up over this... but I also understand that whatever happens, it's not my fault - nor is it my responsibility - to fix things for them. I guess time will tell.
I'm glad you know it is not on you here to solve the issues between them.
Maybe reading "poly hell" together helps him/her start having the conversations they could be having. Either with each other or in therapy.