My neighbor is a therapist and her husband did this to her. He ran off with a much, much younger woman -- right after my neighbor had suffered a serious medical problem.
She tackled it by remaining fairly calm, letting him go, and maintaining low contact. (Although she suffered a lot!) She told me she would correct things but it sounds like she did not overly engage. They maintained some contact but he was far away.
He almost filed for divorce but then ended up returning after six months. She said it took an additional year of "working it out" before things were back on track, and that he was much happier and more settled in the marriage now.
He himself told me he knew what he was doing was irrational, but he felt compelled to do it. He told me though that once he realized his wife truly no longer cared -- it scared him.
However, my neighbor said her husband was much more lucid and less volatile than mine. He was reading Jung, in therapy, and more self-aware.
I wonder sometimes if discussions on this site become polarized because MLC can be so varied as to almost be a spectrum disorder. To this point, I would actually think a great benefit to this site would be to rank MLCers like hurricanes. Some have low durations, surges, and damage whereas others are the opposite.