Employee doesn’t get the promotion, then has to train the new hire: Is there an equal pay issue?

Q: “A male manager retires and his female assistant fills in while a search for his replacement is made. She apparently covers the job adequately in the absence. The company hires a young male (an MBA) to replace the manager. He is hired at a salary higher than hers. She wants equal pay to her manager, as she is basically training him. Is this an FLSA/discrimination issue? Is the company mandated to give equal pay?” - Anonymous, New York