She has no problem putting this on you. If you haven't finished her car, don't. If you have done the work and she is still insisting, it's time to break tie's anyway. You were helping her out and she comes to you again to take care of the ticket. Your car or not. She dated you. Every cars guys girl knows the things on the car that you worry about with the cops (correct me if I'm wrong). Therefore; she knew when she took the car what she was driving. Her problem ultimately, either way. If you pay, are you going to look at her the same or help her in the future? Probably not.
Yeah, it's only $100. Is it worth the trouble either way? $100 or not, she seems like a bit of a mooch. Why loose anymore time or $ with this girl? What do you get out of it?