flirty joke to try and impress or woo this OW into having sex with him
Why would this be acceptable to her?? If the OW were interested, he'd have acted on it by now. Tbh, that she has told him quite clearly she will not sleep with a married man and he still pursues it shows that he is being incredibly disrespectful to both women. Even her comment aboutarried men not leaving their wives sounds more like she is holding a mirror up to his shit behaviour rather than looking for reassurances of how he feels about her.
Of course it's not a joke that's getting out of hand! He's an adult. He's not a silly teenager who isn't aware of how this is coming across!
I have a male friend who sometimes jokes with me that if he and his wife ever split up... but that's it. No sexual comments, no flirting, nothing. His wife knows he says it. He loves his wife and only ever speaks highly and positively about her. It's clear to everyone that he loves her very deeply.
It might not be for everyone but it doesn't impact on their relationahip. That is just a joke.
The second he started criticising his wife, talking sexually, and the fact they were fuckbuddies before he and his wife married? I'm telling you now, none of that is in joking territory!
Why is she not angry? Where are her boundaries and self respect?
But, I agree, ita your friend's issue and not yours.