She is defiitely in the cusp of an eating disorder. Exercise and moderation is always the key, not faddy dieting or starvation. Not ony can they royally mess up your body and have a lasting bad effect but you rarely ever keep the weight off after.
Honesty time. I had a an eating disorder, as did a friend. She starved herself for a time- she wrecked all her teeth, shrunk her stomach, her periods stopped and she has fertility issues most likely because of it. I binge ate and purged. I thought I could control it and was very wrong. I ended up with bad IBS and food issues. I get many anal fissues now thanks to the IBS which was directly caused by my method of purging. Screaming while shitting is not a pleasant thing and it and the IBS severely restrict my social life now, every time i go anywhere-even travelling in the car- I am looking out for the nearest toilet. I too may have fertility issues, have awful stretch marks and struggle sometimes not to binge.
Your concentration will go, you skin will pale and look clammy, you will look ill- not good. People may compliment you for your weight loss as many do, they will say you look good but they will not be looking at your face as they say it, if they were they would not say it. Your periods will stop or become erratic, you will get breathless easy and possibly faint. You will lose the weight you want and feel satisfaction, then in a short time you will gain it again and feel the need to starve. You will never be happy with yourself while doing this and every repeat will lead to more self loathing and belief that you are useless.
Your daughter is looking for what she thinks is a viable short term solution...it will never be short term, she will keep repeating if she gets into doing this. She will think she's in control but actually will have no control. It will mess with her body and mess with her head,she will end up cycling through loathing and loving herself. She needs this honesty.
Perhaps she could try some other kind of exercise, a DVD or something like the 30 day shred? 20 minutes every couple of days, that's all. Far better to exercise then starve herself. Just remind her that if she's exercising she will always gain weight from muscle gain.