I think being a good listener and showing general care and concern without asking questions about her or their relationship would be a safe way to do this. I'd probably try to allow a slow comfortable friendship to develop, and possibly at some point he'll feel comfortable enough to talk about this. But if not, just working towards filling in that emotional space she's leaving would be a good way to build a closer marriage.
Good luck to you!
There is no arriving, ever. It is all a continual becoming.
