I think you are imposing your own values onto her.
This arrangement works for her. Nor does it close the door for her to enter another relationship.
You know, there have been romances written about things like this. Like “Same Time, Next Year”.
Basically, if she is happy with it, why change it? While he is doing this outside of his marriage, there’s no mention on whether his wife is aware. Otherwise, it doesn’t sound like either one is frivolous. There is an emotional connection between them.