I realize I don't know your friend nearly as well as you do, despite your excellent writeup. But might I suggest a simpler explanation? He had the hots for his girlfriend's friend, but never made a move either because a) he was afraid to, b) he didn't want to while dating his girlfriend, or c) he tried but was shot down (either she wasn't interested, or didn't want to steal her friend's bf).

And when you casually got in her pants after just a few days, while he's been lusting after her for months, he got pissed.

IOW, I'd suggest it had nothing to do with associating it with the day he got dumped. But rather the more ordinary reason of a blupilled dude with scarcity mindset who was jealous that you bagged a girl he couldn't get. If there's anything to do with the timing of his breakup, maybe just that precisely when he was single and could shoot his shot with his girl, he finds out you've already stolen the ball.

Years ago, when I was younger, I had a roommate that was far more successful with women than me. We were both desirable men, but he was much more of a player, willing to plate women, and spend ludicrous amounts of time dealing with the drama of juggling several women at the same time. So inevitably he had more women around than me. And once in a while, I would get a pang of envy when I saw a stunning woman in our kitchen the next morning that he had bedded the night before. Sometimes, it was even a woman I was somewhat interested in. We never outright competed against each other (bros before hos!) but my process was much slower than his, so sometimes he'd end up swooping a girl while I was still thinking about how to tell a girl I was interested. (My fault for being slow and dense)

But here's the thing that saved us (we're still good friends, despite now being a thousand miles away): neither of us had a scarcity mentality. I realized that some girls like him, and some girls like me, and there's surprisingly little overlap of girls that would be interested in both of us, due to our different personalities, style, method of dating, etc. Sometimes girls I liked preferred him, sometimes girls he liked preferred me. At the end of the day, as long as both of us believed that even the hottest women were replaceable and less hard to find than a good friend, it was all good and it never caused drama between us.

This is partially why I stopped flirting with women when I was with blue pilled friends. Sometimes, even without trying, or even actively trying to get a woman to be interested in one of my friends instead of me, the girl would circle back to me. Not because I'm some amazing guy, but girls just have their preferences. With red pill friends, this was never a problem: with abundance mentality, all of us knew if it didn't work out with this one, there were plenty of others out there, and women were never worth blowing up good friendships for.

But with blue pill friends, they'd get all sorts of butthurt that I was "stealing" their girl (that they couldn't work up the courage to talk to) and sometimes would stew about it for days. It just wasn't worth it.

I wonder if the same dynamic was happening here. And it would have been double frustrating that you were able to drop her so easily. If, in his mind, he wanted to "properly" LTR this woman, but she preferred a short term fling with you, that's doubly insulting in his mind, and he's taking it out on you. Sort of like when your oneitis that you would marry on the spot, knowingly decides instead to have a drunk ONS with a stranger.

Anyway, thanks for the write-up, it was an interesting read.