AT can't help fix this situation. From the sounds of it, I don't think intense counselling can, either. But I will say that someone changing their lifestyle is not a bad thing and that there are probably more underlying issues that we (or you) don't know about. What I'm saying is that there isn't just one thing making this happen. There never is.
I suggest that even though you care for them, give them their space and don't violate their trust by telling secrets or making suggestions to either one behind the other's back. If they want to save their marraige, they are the only ones that can do it.