

That’s perfectly fair, but don’t expect the original person to want to get back together with you then. It’s not about whether it was ok for him to have sex with that girl, it’s whether or not Rachel was justified to not want to get back together because of it. Which she was. And he knew it, or he wouldn’t have immediately tried to hide it.
The problem is he didn’t stick around to talk out what “a break from us” means. He stormed out without saying anything when she said that. And it’s not really his fault, he says he assumed that meant a full breakup which isn’t unreasonable. But most people know that in reality when you’re in a big fight with your SO, sometimes you say things that you don’t mean the next day so you probably shouldn’t be making huge decisions that will affect the future of your relationship based on them. Assuming you want the relationship to continue which he obviously did.
He’s not a bad person for doing what he did, he’s just an idiot for doing what he did when he wanted to still be with her. Even if it was a full breakup, that doesn’t mean he couldn’t win her back. Sleeping with someone else makes that much harder though.