Honest question here, would you have done the same if it were a man being grabbed by the arm or being blocked? I'm asking because you seem to be equating grabbing or stopping someone from leaving when they're arguing with physical abuse. Did you see him hit her? Was he grabbing her arm REALLY hard? Sometimes, couples have really bad fights, not physical but just really bad reasons for fighting, like maybe one of them was caught cheating, who knows. When one person wants to get away, the obvious thing to do is to let them go their way, we know this with a clear mind but maybe depending on the situation it might not be as clear.
Maybe she caught him cheating.
her: "get away from me you're a fucking asshole!"
him: "baby wait no pls let me explain!"
her: "no, get away i cant believe this, i dont wanna talk!"
him: "no please dont go pls let talk to me!"
I don't know what you saw, and if he was restraining her physically using excessive force and causing her pain, then good on you. Otherwise...idk maybe its a lover's quarrel and you should've minded your own business. If you wouldn't have done that for the male if the situation were reversed, then maybe you let the gender bias influence you. Not trying to start any flame war and I'll probably get down voted to hell in this subreddit, but not every male is immediately abusive especially if you don't know the context of a specific situation. It just seems that if she was in public and she truly needed help and she was in a dangerous situation, she would have made it known and may have been more frantic. From what I read it just seems like you injected yourself into a lover's quarrel. Unless he was hurting her, then I agree.