You should walk away, but pick the mic up as you have not proven your point.  Most employment is "at-will" meaning you can leave or be fired at any time.  Other than actors playing in a movie, only white Americans dress up as black Americans.  An outrageous reaction would be to put her in jail for her actions.  If this 50 year old professor wants to use poor judgment, why would that give her a cause of action to sue?  Is it wrongful termination to not want an "alleged" rapist teaching class?  Is it wrongful termination to have an "alleged" racist teaching class?  No one wants to associate with a rapist or a racist, allegedly or not.  If your actions outside of your employer have an effect on your ability to do your job, your employer has every right to fire you.  Just because not many black Americans live in this community doesn't make it okay to mock black Americans.  The only bigotry is coming from her and those that defend her actions as no big deal.