He did say he thought she wanted to get back with him and he was bumped she didn't ask too. Clearly he also wanted to get back with her. It doesn't matter who your ex is "friends with benefits" with at the time, so long as they keep it monogamous after you get back together. I don't see why he shouldn't have sex all night long when he and her were broken up and he thought it was impossible to get back together even though he wanted to.ヾ(☆▽☆)
You're only seeing things from HIS perspective. But as a woman, things would be game over for me if I actually had to hear my ex-lover, who I still had feelings for, have sex with someone else. ALL.NIGHT.LONG. It's one thing to know he has a lover; it's a whole nother thing to actually hear the person you care for make love/fuck someone else. Broken up or not, that shit would hurt too much if you still had lingering affections for that person. She realized it too and that's why she left without a word. He'd moved on and now it was time for her to move on as well. And let's be honest, he wasn't too broken up about it. He had a moment of regret and then it faded as fast as it came. He just didn't love her anymore.

Girl didn't lose to a dick, had she just voiced out her feelings they could have gotten back together lol