How do we know she would have chosen to do something else when she didn't?
Short answer: ... again, NO.
Longer answer: You don't.
Regardless of the answer, does it matter at all whether or not humans have genuine free-will? We all think we have free will. We all act as if we possess free will. Questioning whether another person has free will is an exercise in futility, pure and simple. We cannot know if a person would have chosen A or B if given the chance again. And even if the person chose A
every single time it still does not prove or disprove whether that person has free will. It is true that a person, their concept of themselves and their actions are created by the environment they have grown up in, combined with minute differences in brain formation... but that does not negate or prove free will. It simply allows that the persons actions can, if you know enough about that person, be predicted with relatively strong accuracy. That fact still does not prove or disprove the person possesses agency.