In our state, you are required by law to be separated for a year and a day before you can file for divorce. Hence the 366 days. So literally, they never lived together. As I said, it was a very strange situation all together.
Long story short, they were friends (and actually still are), she was there for him when no one else was (long before we met). He felt beholden to her in a sense? I'm not sure that's the best way to put it, but it's the best I've got.
She knew he was seeing me, knew things were getting serious between us, but I guess she held out hope once they actually got married maybe he would see her the way she saw him? It never happened. And I know it's shitty of him to have taken it that far, but at the time, I guess he thought it would have been worse to back away.
His actions were very selfish.
But here we are, 6 years later.