Really sorry to hear about this mate.
First of all as nipper suggested get the boys and go have a word with the person who caused this.
But on a more serious note, I could understand the lemon and viniger for a week to detox but from the line "she has told me that she wont go baelemic again" I assume she has had issues with her wieght before so this seems more like she could be relapsing, which is what you don't want.
The first thing I would do is talk to her parents. I don't know what they are like but I'm sure if its to do with their daughter's health they would want to know, also don't feel guilty about going behind her back and talking to her parents as she should understand why you are doing it. The other reason for doing this is, if I've assumed right and she has had problems before, then her parents would have experianced this with her and might know what to do. Also this would give her more support.
After a quick look on the net and a few BMI sites (Body Mass Index) they say a person of 6" should be between 10st 7 to 13st 7. You haven't put what wieght she is now but 9st target is way under what she should be and loosing 6lb in 7 days, and over 4 weeks has lost 3 inches on her waist is not good for anyone unles they have been put on a diet and being regulaly checked on by a Doctor or qualified dietrician. Also this could cause her to loose her assets aswell (natural curves, bust, etc) as when you loose weight you loose these aswell, a good example of this was when Gerri Haliwell done her extreme dieting.
I kinda of know what you mean about an emotional rollercoster, I had a bit of a bad patch with my other half (who I've been with for 5yrs and good mates for 2yrs) a few years back, it wasn't about wieght it was about our relationship, but as I said I can relate to how you feel as its like a double whammy, you are worried/upset as a BF and as a friend which makes it twice as hard, all I can say on that is to be strong and stand by her and help her through it, it might take weeks or months but if anything it will make your relationship stronger.
But as I said before the first thing you should do is go and talk to her parents , I know it seems like your betrying her but your not as this is very serious and needs sorting.
Hope thats been of some help and hope you get this sorted