jhunter,
i too have looked up the details of your daughter's weight. At 5'9" and weighing in at 120 lbs., she is on the verge of being anorexic. Her bmi is 17.76. To be classified as being anorexic, the bmi stands at under 17.5 and to be classified as underweight, the bmi stands at under 19. She is only right on the edge to being classified as anorexic. As slowcadence said, early intervention is key in preventing the illness from getting any worse. She needs help now, whether or not she likes it. This can get out of hand quickly, and before she knows it, she'll either be in the hospital being tube fed in order to keep her alive, or she won't be with you anymore. Anorexia is a VERY serious illness. Getting better takes time, and right now, time is not on your side. Get her help as soon as possible love. I have dealt with anorexia/bulimia for the past 11 years. Right now, it's under control, and has been for the past year. If you have any questions or concerns, PLEASE don't hesitate to pm me. My thoughts are with you hun.
sniffles