I've been following this thread and I think it's necessary to point out that I think Hermione would be very much understanding of an eating disorder. Disordered eating is a control problem linked inextricably with depression and anxiety, and from what we know about Hermione, she is a veritable control FREAK.
Many times, an eating disorder has nothing to do at all with your body image, but rather the fact that you have so little in your life that you have control over. The way you look is something that is most definitely yours and something that you have direct control over.
In my early teen years, I had a lot of disordered eating and a binge/purge cycle. I think it's incredibly important to make sure that you explore the psychological aspects of this disease (because it is a disease) and remain sensitive to the actual causes. I think Lily's aunt Hermione would be a great support system and advocate for her, because she so intimately understands what it's like to need control, even if that's not the method she herself chose when she was in school.
Feel free to PM me if you want help writing the disease, I'm very forthcoming about my experiences