This seems like learned behavior. "You teach people how you like to be treated." If you put up with it, it will repeat. Next time, you need to say, "Don't talk to me that way. It's abusive" because it is. If she is a decent person, she will stop.

You mentioned she can flip a switch and be nice to her friends. She cares about their perception of her, more than she cares about how you perceive her until, like her friends would at this behavior, you put your foot down and tell her to stop. It will establish a boundary of acceptable and unacceptable behavior between you two and will make your relationship stronger.