How do you help someone with amenorrhea get back on a regular schedule?

Depends on cause: There are many causes of amenorrhea, and thus treatment depends on cause. If due to pcos, then weight loss, exercise, lifestyle change and medications can help. If due to athlete's triad (underweight), then increasing body fat percentage may help. If due to other hormonal (thyroid, prolactin, androgen, etc), or anatomic causes (pituitary, ovaries, uterus, etc), then need to address specific issue.