From Chofetz Chaim Hilchot Rechilut: Chapter 9
Certainly if the observer knows that a deception is in progress he should warn the customer. [One of the examples:]
C. If the shopkeeper is inflating the price. This is only if the price is inflated more by than one sixth (i.e. one fifth of the correct price); the Chafetz Chaim explains that overcharging by less than one sixth may be permissible.
Personally, as for menchlichkeit, I don’t see anything wrong with what she did. In her eyes, she was doing you a favor and while you didn’t appreciate it, a lot of people would have.