Not even close. Two things are apparent here: that he didn't hit her with his full strength, and he didn't continue the attack beyond that one punch.
You, on the other hand, have to magically prove that a softer punch (exactly how much softer we're talking here, who knows but you) would have stopped her from attacking him.
Good luck with that.
I don't have to "magically" prove anything, you simply need to abandon this absurd notion that punching someone who punched you necessarily meets every legal requirement for responding proportionally.