IMO, the Mid should be discussing this weight/body fat issue with her chain of command and/or classmates. Her company officer and senior enlisted leader can help her to understand the exact requirements. As I understand it, if a person is fit, but outside of standard height/weight limits, they can have a body mass index measurement done. If their body fat is within acceptable limits, they may still be good to go. Some athletes who have higher than normal muscle mass may exceed the weight limit for their height, but they may not be able to lose much weight, if they are already at a low level of body fat.
If your friend is already looking at options outside USNA with six months available to lose 15 pounds, I suspect that her decision to leave may already be made, and the weight issue may be her way to explain the decision to others. If she truly wants to remain, I am sure that those around USNA will help her to overcome this obstacle.