@melissa123 It's so kind of you to bring your concern about your friend's daughter here. Is the mother also concerned?

I agree with the recommendations of @dlydailyhope and @roch. Spending 3 hours on an ellipitical is excessive exercise and then the report of the restricted diet leads me to think the daughter is experiencing problems with her body image and perhaps an eating disorder. I've read many accounts of female athletes such as gymnasts, x-country skiers, and runners who experience amenorrhea but it does not sound like this young woman is in training for a sport. Is that correct?

The place to start is with the primary care doctor. Since the daughter is 21-years-old she is an adult and so the mother cannot bring the daughter to an appointment as she might with a young child.

Have you discussed these recommendations from the members here with your friend?