It sounded like she changed her mind on the kids thing - or at least they didn't discuss it as much as they should have.
The sense I got was that it was status related and he was pretty close to right. She didn't like being with someone who wasn't working, didn't like telling her friends her SO didn't work, didn't like not having things to talk about, etc. He was low status compared to her friends.
Eventually she cheated on him - I think that's wrong, but the motivation behind it is not unpredictable or uncommon: https://samharris.org/podcasts/254-mating-strategies-earthli...
The new relationship is different in part because his SO is a librarian with a small income, there isn't as much of a status divide and he is high status in that match given his high net worth.
There's a lot of evidence that women tend to select people at their level or higher for mates: http://rationallyspeakingpodcast.org/216-being-a-transhumani...
A lot of this feels like a narrative on top of that core issue without recognizing the underlying problem.
Obviously things are more complex than this at the individual level, but it feels like a lot of talking around the core issue without recognizing it.
I'm not making morality judgements on any of this (except perhaps the cheating), but it's good to recognize the pragmatic reality of the world you're in when making these kinds of decisions. If you decide to retire early, it'll likely affect your relationship opportunities in some ways you may not understand.