I regret to inform you that you cannot FORCE an adult into inpatient treatment unless there is:
- evidence of trying to harm herself
- evidence of trying to harm someone else
- evidence of significantly violating the rights of others
- frank incompetence
...you would likely need to prove these to be true in a court of law to REQUIRE someone to go. (If your the person in question gets to the point in their eating disorder that they need to be hospitalized for MEDICAL reasons, this would be the perfect opportunity to get them the psychiatric/psychological help they need as well.)
I realize that this is not particularly encouraging news for you - as a friend or family member who is suffering. Fortunately for all of us, we are given certain rights and responsibilities for ourselves - including the right to make poor decisions about our health and well-being.
What may be helpful for you is for you to go to one of the treatment centers where your friend/family member attended and to ask one of the licensed mental health professionals for some support on how to cope with having a sick person in your life. They will help you not only to explore what you can do for her... but also what you can't - and how to take care of yourself while your friend/family member deals with her illness. Remember - you can't be of help to anyone if you're not taking care of yourself first. (The flight attendants always tell us to put on our oxygen masks first before helping anybody else with theirs!)
I wish you (and your friend/family member) all the best.