Partner getting Divorced
I've been dating my partner for about six months. He two other partners, one of which being his wife of many years. They'd been in an open relationship for years (their definition being just sex) and about a year ago they decided to open that to poly (their definition being relationships with the sex). She's now decided that she wants to go back to monogamy and have him go back to the person he used to be. The chances are very slim that this is going to happen, he has other people in his life now and he feels he would be unhappy going back to that lifestyle. There are other issues of course that have grown from their time together, but the poly thing is the big one. They've essentially decided it's over and are just getting to the point of officially ending things.
My question, I suppose, is how do you support a partner who is facing the end of a marriage? He moved to my city about a year ago and doesn't have many people to stay with, so we've basically decided that he'll move in with me (if and when it happens). This can either be temporary or permanent depending on how we get on in that capacity. I love him and want to help him through this but I know it's going to be a shit time for him. I have no idea what to expect and I'm concerned about walking the line between carrying on with our relationship and allowing him the time and space to grieve for his.
I realize every situation is dependent on the people involved, but if anyone has dealt with helping a partner through a breakup and has advice I would appreciate some insight.