This was apparently not the first time there had been infidelity in the marriage.
There have been affairs (on both sides) in the marriage over the years, but this is the first time he told her that he was in love with the other woman and that he was leaving her.
The Husband had a long-time affair with this woman.
This was no fling with a groupie! [Husband] and [Mistress] have been together for a long time. He was absolutely in love with her. He wrote her love poems and songs and love letters. They were living together! He was ready to file for divorce from [the wife].
When the wife found out that he was going to file for divorce, she flipped out.
When the divorce news went public, it would make her look bad. [Her Husband] says that is what she cares most about: Her identity, her money, her reputation. He is actually a little afraid of her.
The Wife is much stronger than The Husband. She quickly reeled him back in and crafted a story.
She is not lying to protect him. She is lying to protect herself. She said that he never loved [The Mistress] (absolute rubbish), that she kicked him out of the house (he was already living with [The Mistress]), and that he had a sex addiction (more rubbish). He is not a strong man, though, so he capitulated.
So it sounds like she told these lies to save face. Not his face. Her face. Well, not her original face, of course. That was gone long ago.
[Optional] Did you believe The Wife’s version of this story?