I was once the anorexic daughter that parents worry about. Now that I'm recovered from anorexia and can look at the situation clearly, it's easy for me to articulate what might be needed and wanted—from the point of view of the sufferer.
Okay, let's be honest; both parties are suffering.
I feel the best thing you can do for your daughter is to love her and support her, which you obviously do if you are reading this. Try to understand her. She is probably feeling as though not many people do, but all you have to do is let her know that you WANT to understand. That you care enough about her to want to help her in any way you can. And that you will always be there for her when and if she needs you.
Don't be obtrusive. Don't be harsh. Sometimes it takes a big wake up call to make someone snap to the realization that they are really damaging themselves, but try to be the rock she can turn to when she is faltering. There are plenty of ways to show her the error of her ways without punishing her, condemning her, or making her feel worse.
My parents did good things and bad things. But it's hard for parents—and for anyone—to know how to deal with an eating disordered child, especially when that child is actually a young woman and not a child any longer.
Ask her if she wants you to do anything. Ask her if she wants freedom, support, more recovery resources, info about a support group, a shoulder to cry on, etc. Perhaps she wants nothing. Perhaps she doesn't know what she wants. But if it was me (and it was me once), I would greatly appreciate having my mother or father pose those questions to me.
If she's been in treatment: Just because a treatment place did not work for her in the past doesn't mean she can't get help or will be resistant to other forms of support. One thing that really helped me while I was a college student was going to a support group with other young women who were facing the same problems. We learned to want to help each other, which in turn helped us to help ourselves. If she cares about the well-being of other girls like her, she may invest time in a path towards recovery and soon start to hear what they are saying—and use what she is telling them on herself and her own situation.
Sometimes eating disorder support groups are not publicized and are hard to discover. Ask your local hospitals, do a search online, or contact an eating disorder specialist for info. Don't push…but it's always worth keeping in the back of your mind.
I really feel for your situation. I have a very good relationship with my mother and I know how helpless she felt when I was very ill.
Be there to listen to your daughter, but if she doesn't want to talk, don't press. All you can do is try your best to help her and the rest is up to her. No one can make her do anything—and if they do, it will only be temporary. She needs to make a choice for herself and set goals.
But you can support her in these goals.
Show her how great life can be without her eating disorder instead of showing her how bad her life is with it.