Sounds like you had the situation totally assessed. Were you there?
The long and short of it. She had her hands on the police officers shoulders appearing to try an obstruct the officer from apprehending the other person in the booth. The officer then swung at the girls face, open handed. Afterwards she throws a flurry of fists at the officer. There was another officer there and he should've handled the woman who was obstructing justice. Officer #1 couldn't count on Officer #2 doing his job, and was forced to act and decided to punch the girl in the face. Lets look at the other options. He could've tried to grab her while she's swinging at him, risking injury to himself. He could've gotten out his baton or nightstick and started swinging it risking property damage (if he broke the light for example) or injuring the female suspect. He could've tried mace or a tazer and risked collateral damage by macing bystanders or God forbid missing the girl with the tazer. Punching the girl actually seemed like one of the better options considering the options within the confines of that situation. That place was too busy and crowded to wait for backup.