Dealing with eating disorders in the family, my heart is sinking reading this.
The parallels with addiction are many, the most prominent one being the utter lack of awareness of suffering from a disease. With the malnourishment of an ED, her brain is simply not able to function well enough to make decisions for herself -- and that ability won't return until she is re-fed and has maintained a healthy weight (which is not what the government calls "healthy weight" but usually about 10+ pounds more than the top of their range) for a while.
I lived with an alcoholic for 20 years. I have to tell you that two years with a child with an eating disorder aged and wore me out more than those 20.
A lot of my friends in the eating disorder family community find Al-Anon to be helpful -- to work on emotionally detaching and staying on your side of the street. Which you are doing a fantastic job of.
If he were to decide that he wants to educate himself regarding eating disorders, let me know and I can point him to sound sources. The treatment modalities have changed a lot; eating disorders are now recognized as brain-based, biological, largely hereditary diseases that can be "turned on" (like flipping a switch) by reducing calories or by a traumatic event.
Sorry to write a novel about it but I'm living and breathing eating disorders right now and it is very difficult to find qualified care, especially for an adult who can't be involuntarily committed by parents.