The story of these men is always that they are going to leave their wife, which they obviously won't.

Agree - so presumably the author believed what she wanted to believe, or didn't really care.

An estranged wife though.

He claimed. He was still married to her years later, so not that estranged.

Plenty of decent single men out there, she went for the married one with kids.

Not disputing the guy's behaviour seems appalling; so her due diligence on his character and his actual single status appears to have been poor.