The way I see it is that the girl intervening with the officer making an arrest (justified or not, though probably not justified because the last time I checked, you don't/shouldn't get arrested or searched for jaywalking) could justifiably be placed under arrest. Cops aren't regular people outside of a bar that you can scuffle with if they say or do something wrong, you just can't. So then, the chick that got punched would rightly be placed under arrest for intervening and even touching the officer.
That being said, the punch (a manifestation of the police officer's perceived power to subdue people under arrest) is a clear-cut example of excessive force. This wasn't some 300-pound ex football star or a raging lunatic, this was a teenage girl, and the force used to subdue the situation is not befitting of the person being taken under arrest. If the intervening party were an angry male approaching with clenched fists, we'd have a different story, but there is no way you can justify using that kind of force on a teenage girl who most likely has no training in unarmed combat and is in no way a threat to anyone's life (she wasn't even striking at the officer, to say nothing of the fact that even if she were, she's a teenage girl).
I know the Seattle pigs have come out in defense of the officer with things like "the officer was in fear for his life" and "he was surrounded by a hostile crowd", but anyone that gives the video even a cursory watch can see that neither of these things are true. The "attacker" in this situation was a very loud and angry teenage girl, and the "crowd" was doing nothing but watching the goings on from at least 50 feet away and in no way were they attempting to intervene or even making moves to do so.
I doubt anyone would say the girl that got punched wasn't doing anything wrong, but it is clear that the way in which the situation was handled involved excessive force on the part of the officer, and now this girl's life will most certainly ruined by the assault charges that are sure to come.
Maybe she'll get a nice settlement from the city, but maybe she won't. What I do believe is that nothing even resembling a punishment will be handed down to the cop, save for maybe a cushy desk job for 6 months until everything blows over and he'll be sent back out on the beat. It's pathetic.