if she is seriously discussing this issue in public, you sure can ask her to stop, she then may get personal and start to accuse you or even get physical, well then that's ur change to express your own opinion and/or defend for your self.
I once saw it in a forum discussing something similar like this. the input we had was.
Q. some1 offending the law, do we need to stop it?
A. Depending on the scenario, if you are in total control or a law enforcement officer and have been trained to protect the law, yes, go ahead and stop it.
for civilian, our responsibility is to obey the law, and protect ourselves from being harmed, protecting the law and enforcing it is not part of our civilian responsibility.
so if you feel u r offended, call police officer or contact authority to deal with it. or if you feel being abused and you have been trained in confronting these issues, you can stand up and protect yourself for sure.