She hit her ex in the face, and then laughed about it? Noooope. I'd drop her after hearing that. Hitting her back could've put him in jail. So it's not that he can't, but if he did, there are serious repercussions if he does. She doesn't get that.
Big red flag imo. If she hit him, she's probably unpredictable. Guard up OP, if you decide to stay with her.