Also have a friend in my group who hits guys on the arm, as well as other terrible behaviour.

One time at a party at my house, a guy hit her back, with the same amount of force (friendly tap) and in the same place, and she flipped her shit. Full on attacked the dude. Scratched him across the face. Which forced him to restrain her by grabbing her wrists. Then of course the knee to the nuts comes out. A few of us, her boyfriend, brother, and myself broke it up and took her outside to calm down. (Note: we didn't have to restrain the guy, even though he was the only one to get hurt with aching nuts and a bleeding face) She basically then straight up told us we should beat the shit out of him, like the kind of kicking you'd dish out to a rapist or child molester. 'Aren't you going to fuck him up?' None of us complied and even the brother and the boyfriend were like 'No. You're way out of line. We saw what happened. Sit here and stop starting shit with people'. And then the guy left the party because he thought it would be best to diffuse the situation and even apologised to me as the host. Of course I let him know that he was not in any way to feel responsible or unwelcome but he didn't really feel like partying after being assaulted. Fair enough. Oh and on the way out the girl screamed abuse at him and called him a coward etc.

And she never managed to see that she might have been in the wrong. That firstly, you don't have the right to touch people. And if you joke around with someone you're inviting them to joke around too. Secondly, that even though she was annoyed at being hit back her response was completely disproportionate to the nature of the exchange. And her response actually constituted a serious assault.

Since then, and other bat shit crazy things I'm not really her friend anymore. The boyfriend also kicked her to the curb. Sadly for the brother he is still the brother.