I dated a girl who was quite bad with bulimia for a while. This is going back a few years before there was all of the awareness that there is now (not saying there was no awareness at all, but it wasn't all over the media the way it is these days)... it would have been about 95 or so.
She was honest enough to admit it to me before there were any telltale signs in her behaviour. That must have really taken some doing. I was completely ignorant about eating disorders at the time, and it prompted me to do loads of reading. It also in a stupid way brought us closer together. She felt she could share her innermost thoughts and feelings with me, and I felt I could ask her questions... that sort of thing.
One thing you should not underestimate though (just as a word of advice) is how good sufferers are at hiding it. Not saying it's done to be nasty or dceitful or anything like that, more often than not these girls are hiding things from themselves, as much as from the outside world. Convincing themselves they are OK because of xyz reason is a big part of the problem with this stuff.
I'm ashamed to say ultimately it did end my relationship with this girl for exactly that reason. Despite the openness and the talking and the support, I'm sorry to say the outward indications were drastically different to what was really going on with her. To my shame, I found I just couldn't cope. This thing is an iceberg. The bit you see is just the 10% tip poking out above the water.
You are obviously concerned enough to think there's a problem, and from my own experience, that makes it a dead cert. I can't help as to what is the best thing for you to do, but please don't do nothing because you think you're being over cautious, or seeing things which aren't there.
One question though, does she take food supplements and nutrient/vitamin pills and stuff? My ex had a kitchen cupboard like an apothecary's shop and was banging all manner of herbal this and supplement that from these massive bulk-buy jars of pills. At the time, I remember idiotically thinking it was a good thing... that she obviously had a handle on it... I used to badger her to take her pills & make a joke out of it. Little did I know...
I can't promise to be any help at all, but gladly will help in any way if I can. Good luck. This girl is lucky to have people like you around her.