I can tell you that you probably have a right to be scared about being sued. My mother was an anesthetist, and she once got sued by someone after she put in an IV drip in order to save somebody's life because their hand hurt afterward. Go figure, putting a needle in a vein might sting a bit.
The good news is that frivolous lawsuits like this are usually thrown out straight away without causing any harm to the person being sued. Also, dickwad people like this are in a minority. But, honestly, I really can't see how either choice you could have made was more appropriate than the other. Both options were equally reasonable, given the circumstances. Seeing as her friends were there, though, you could have gotten their permission to help, which essentially protects you by having those witnesses on your side, but, hey, I can't say I would have known what to do if a situation like that suddenly occurred.
Don't beat yourself up about it.