This won't be popular, but I for one would still like to hear CB's side of the story. I know of too many cases of enraged or crazed girlfriends grabbing stearing wheels, attacking with knives, etc. to say that there's "never" a good reason to hit a woman.
That said, whatever led up to this... those two shouldn't be together. They get back together and one of them might end up dead next time.