From what I understand, losing a period (secondary amenorrhea) is due to nutrition deficiencies (including insufficient caloric intake), low levels of body fat, and/or excessive exercise (assuming it is diet/fitness related). I also lost my period about 2.5 years ago, and I haven't gotten it back, but mine is likely due to low body fat. Even after maintaining a diet of 2000+ calories with at least 30% fat for a few months, nothing has happened. I've lost my period in the past and it didn't return until I'd gained fat.
Tell her to up her calories. Eat more. Carbs, fat, protein, whatever. Find a better gynecologist. And if it doesn't come back within a few months, she probably needs to regain some of the weight she lost.
"Humans do have a knack of choosing precisely those things that are worst for them" -