Pdx has it down, douchebags all around.
To the guy's (limited) credit, he tried repeatedly to walk away from the situation. His girlfriend was running her mouth like a punk, acts like a punk, and looks like a man. She also grabbed, pushed and shoved him repeatedly.
Notice how quiet she got once he punched her in the face. If she would have known that was a possibility in the beginning, she would have handled herself better.
If I'm chunky-chick, and I see him kick through the car window of the girl he LIKES, that would be my first clue that I might be in for a bad ending if I don't pour myself a cup of stfu.