First off, you didn't cause this. Whatever causes Anorexia Nervosa is already there, waiting to be triggered by something -- but that doesn't mean it was triggered by you in any way. It just means that the timing was coincidental.

As for what you can do, if anything, that's hard. She probably does know that she's in trouble, but that doesn't mean that she can admit it or do anything about it. What I would suggest to you depends a lot on the situation -- is she livinng with her parents? Alone? Dorms? Working? Going to school? You know, the practical things. They count, because there are things that you can try, but what those things are will be different in all those situations.

Meanwhile, educate yourself. A good place to start is reading the information available at sites such as www.somethingfishy.org, or www.anad.org. Learn what you can from those sites, and see if they have any suggestions for someone in your situation.

Good luck.