I say just listen. Try not to make too many statements supporting one or the other, but listen to both equally.

I have a somewhat similar situation at the moment, except that the guy is a much closer friend than his wife. To the point that he is moving into our house for a few months until things smooth out a bit. I struggled with the decision to offer our space, as I've been stuck in the middle of other people's break-ups before, and its not fun. However, the benefits to us and him of being at our place outweighed the pitfalls, so he's moving in this week. Mostly he just needs a friendly ear to talk things out with. I'll do my best to not make judgements about the situation. Hopefully, it will remain amicable.