Um yeah... sorry, but not entirely the girls fault here. Sure, she should have requested safety gear before riding on the back of her boyfriend's bike... But that smacktard shouldn't have been pulling a wheely with her on the back. It's the responsiblity of the driver to provide proper gear for his or her passenger. That means more than just a helmet. So, don't put this all on her for wanting to go for a ride on her boyfriend's motorcycle (or even her friend's.. etc...). She would have been a bit better off if she had a denim jacket along with those jeans. I still can't believe that guy popping a wheely without feeling his passengers arms in a bearhug around his waste. Stunt when your in a parking lot by yourself with noone elses life or limb at stake but your own. Not on a busy highway with a passenger... She's fortunate she didn't get run over by a car behind.
Going frame by frame looks like he likely smacked the back of his helmet into the front of hers (she was leaning tight into his back before he popped the wheely). This likely forced her hands loose, and that's why you saw them slipping off. It doesn't look like she was trying to wave. Looks more like she's trying to hold on for dear life.
edit* -- Okay.. the road was basically empty of traffic... good... but the stunt was still poor judgement, and even poorer luck for his passenger.