I think the real issue is laziness.

If the guy really wanted to save his marriage - a confrontation would have been healthy. Basically, tell her she needs to take better care of herself. If the reason she's not is because she is overwhelmed by taking care of kids, etc., then he could offer to watch the kids for her or do more around the house so she has the energy to work out.

But it takes energy and some work to address issues in any relationship, whether it's a friendship, a marriage, or even the relationship a dog. It's a lot easier to move on to something new and exciting rather than work on what you have. What people don't get - the new thing will also have issues, you just don't see them until you are in the midst of it.