I agree with Venetia.
If the person is legally independent then they can make their own personal medical decisions. I'm an adult if I get into a car crash, and my arm is broken, I don't have to stay in the hopsital if I don't want to. I can check myself out. The doctors, the nurses they can advise me that it would be a bad idea to walk out with a broken, untreated arm, but in the end it's my decision (and I wonder with insurance costs, how many people do make this decision). So if someone is in some medical facility, I do not believe it is ethical to force any medical treatment on a patient.
Now, I think that when it comes to bystanders like family and friends that they have an obligation to try to help their friend/family out. People do stage interventions and try to push their friend in the right direction. However in the end, no matter how much they want their loved one to change, if their loved one doesn't want to change they won't.
There was a show on one of the Discovery Channels a while back; I'm not sure of the title but it was something along the lines of "I Eat 30,000 Calories A Day". One of the most horrific shows I've ever watched in my life. It followed several people who have an addiction to eating (I don't know if it's a medical term, but I'm not sure how one can eat that much if there isn't some sort of mental defect). Some people seem to have some success once they accepted medical treatments. Others went through with it but refused to change.
There was one person in particularly who made my jaw drop. He was so obese he could not get out of bed without massive effort. So he pretty much lived in his bed. He could not shower himself. Could not go to the bathroom. The documentary continued to show his family cooking massive amounts of food so he could get his daily fix. Eventually the question came up to the mother of this man: Why are you feeding him so much food when you know it's killing him?
The mom shook her head and you could tell it's a question she probably asks herself a lot, but she's already resigned herself to the fact that. If no one is there to feed him, he'd get a phone and order out food. She's come to the compromise that cooking ginormous amounts of food would be healthier than overprocessed fast food.
It's really hard to be in that situation, but being an adult comes with the responsibility and the autonomy. Nothing can really change that. Even in cases were someone is physically unable to make a medical decision (e.g. being in a coma), if they wrote a living will, then they still have the final decision.