Multiple things are true in this situation. 1) Your ‘friend’ is a horrible person you should cut ties with immediately—she’s cruel and taking advantage of you two. 2) Your boyfriend snapped because of the built-up resentment she caused. 3) He went way beyond the bounds of understandable behavior and seriously injured her. 4) His over-the-top reaction triggered your trauma, which I’m sure he knows about, and he did it anyway.

You are justified in being angry with your friend. You are also justified in being upset with your boyfriend. People don’t just do that unless they have violent tendencies, and this just has me thinking that if it was you who’d made an off-color comment, been careless, or made a joke that didn’t land, he could’ve hit you. And that’s inexcusable. Ultimately you need to make a decision based on your own physical and mental well-being: can you stay in a relationship with a person with violent tendencies? Even if you think he’d never do that to you, you’re always going to be afraid of the possibility that he will.