There's some wisdom in your posts fizzzness, but I'd baulk at the notion that affairs are 'involuntary' and 'not the person's fault' because neither is true.
What is true and I'd agree with, is that everyone's vulnerable to an ego boost from someone they find attractive and that because there's this insane societal default that people in good relationships won't even feel a tweak of temptation, coupled with the belief that all new friendships are completely unthreatening, people kid themselves all the time that they (and their relationships) are not at risk.
So I can go along with the idea that this bloke had been deluding himself that this was just a nice friendship until the OP told him how she felt.
After that though, even the fact that his own daughter had noticed his obsession didn't move him. He went straight out to talk to this woman.
So he knows.
Unfortunately, what he doesn't know is that his partner values herself too highly to put up with it. She's probably just as much a victim of this nonsense about the relationship being fine and not coming across as controlling and possessive - and as usual, the 'cool dude' posts won't help with any of that.
It's only when he bloody knows that he's risking the loss of a 14 year relationship and his own kids' respect, that he'll make a different choice.
But if the OP sits back, lets this take its course, tries to convince herself that this friendship means no harm and even starts to feel sorry for her partner's lovesickness, what respect do you think her partner will have for her long term?