I believe that under 18s can be forced into hospital for medical treatment, although her parents would need to be backing a doctor for this to happen. As eating disorders are usually considered to be mental disorders, then she can be sectioned (against her will) when she turns 18.
Obviously it is difficult if you have to force someone into treatment, but in a lot of people it is literally the difference between life and death.
I realise you're not close to her any more, but you can still try to speak to people who can help. If you have a guidance teacher or similar at school, they should probably be a first point of call, as they can contact the girls parents and talk to them on a more serious level than you may be able to.
A lot of women are paranoid about their weight, and given that most models and actresses are size 8 and smaller, it can put pressure on a young girl to want to be the same. If someone sees a size 6 as perfect in their mind (and there is a big difference betweent that and a 10), they will feel fat and very depressed about their figure.