Well, the video doesn't show anything leading up to that point, which is unfortunate because we have a very vague idea of why the officer was detaining them. All we see is the one girl obviously resisting arrest, and another girl stepping in.
Okay, first of all, do I think that the officer over reacted? Yes. Absolutely.
I do, however, think that the girl was resisting arrest, which is a no no, and the other girl should not have approached the officer. I'm not an expert on police procedures, but I think he would have been within his rights to taze both of them. Punching the girl in the face seemed very extreme, but she did put her hands on him while he was trying to make an arrest.
I think this is a case of an officer abusing his power, overreacting to a situation, and then taking things to an extreme level that they should not have reached. He is at fault and I think he should be punished. I also think that the girl should not have resisted arrest. She should have let him cuff her and then sued the shit out the department for wrongful arrest. I also think that her friend was very stupid for trying to interject. I might be wrong, but I think he was well within his rights to protect himself given that he was surrounded by people, and they were closing in on him. He was alone, and he was struggling with a "suspect". I'm not sure what police procedure is, but I do know that they are allowed to protect themselves if they feel threatened.