"One day he said that once I'm out of the picture and once his divorce goes through, he plans on pursuing a romantic endeavour with her."
Wait... what? He's making plans for when "you're out of the picture"? This is not someone I would be comfortable with my partner getting involved with. And she said she'll probably sleep with him even though your agreement is that neither of you will get involved with someone new if the other isn't comfortable with that person? And you don't trust her to resist his charms either way?
Um, forget about trusting him. Where is the trust between you two?
This whole situation seems fraught with issues. Am I the only one seeing this?
So, sure, there's nothing that says a poly person can't have a one night stand. There is, however, something saying that no one who knows this dude's MO should have a one night stand with him, and it's called common sense.