His wife developed a type of post-partum depression after their 3rd child and he took over most of the family chores. He supported a new friendship that she developed over the internet when an old HS friend emailed her. He even encouraged it as she became happy and 'normal' again. One day, on a weekend, he came home from work and asked the kids what they did for the day. "Nothing...mom was on the computer all day." The rest, of course, is history as we all know here.
Ugh. You're right -- that story is all-too-frequent. It's a common mistake to think that we can just "be supportive" and our wayward spouse will suddenly "see the light" and come running back.