Unfortunately for your boss, the company does have an obligation to make sure that you don't work in a "hostile" environment, even if its a customer.
Ask your boss to just have someone else wait on her or deal with her when she comes in. Make yourself useful in the backroom, or go have a smoke or something. No need for a confrontation with a customer right away.
If she doesn't back down in a couple of weeks, have your boss quietly pull her aside and tell her that the employees are not allowed to have "personal conversations" with customers, or some crap like that.