Demosthenes2:

... in practice it would be impossible to prove ...

... All they really need to do is ...

Real cases are determined by the real facts on the ground. There is no "magic bullet" for the employer, especially if they do have a gender bias. That's why I recommend that she consult a (free) lawyer to go over her options. Discrimination cases are made, evaluated, litigated and settled all the time.

As far as a "market-based" solution goes, what's wrong with appealing to fairness? If it works, you get "what you're worth" based on that appeal. Most employers want to treat their employees fairly, as it is in their long-term best interest.