How good a friend is she? Of course you can just 'drop' her, but in a similar situation I have said 'Sorry, I feel really strongly about this, and I now feel really awkward....'
This sometimes works because the other person doesn't want you to feel 'awkward'.
I would though, wait until the situation arises again - then if a group of you are sorting lifts out, you can say it quietly to her.
I do agree that she is quite in the wrong, and I too think (without looking it up!) that both she and the taxi driver are liable. And you need to think of your own safety. I too, knew a family where the backseat passengers were killed as M0nica described.