It seems like the one and only solution is to talk - which he's refusing to do.
Your daughter is in an awful situation. She knows there's a problem, doesn't know what it is, knows what she should do about it, but can't.
Whatever the issue is, and however much her husband doesn't want to communicate, things will only get worse if he doesn't open up. And that's really, really unfair - she's left to bear the burden and, basically, shut up about it.
Personally, I think it's time for an ultimatum. Either he talks, or she walks. Does he really expect her to simply accept this without any explanation? However difficult it might be for him to open up, he's being very, very selfish.