This is an excellent question. My twin sister struggled for a while (and still does) with severe anorexia and bulimia. She is at a healthy weight now but she used to be so skinny that she fainted in public.

To support her at home after she was discharged from the hospital, we were told to encourage her to eat but don't get mad if she has trouble eating. Also she struggled with binging and purging so we were told to also not get frustrated with her if she binged.

Also, the nurses told us to keep her in sight for 1 hour after every meal so she can't purge. (That's what they did at the hospital, none of the patients could go to the bathroom after eating unless they talked the whole time to a staff outside to prove they weren't throwing up).

But obviously you might not be able to control someone like a hospital does. If it gets really serious, reccomend them to get professional help because you can't cure someone's eating disorder on your own. It takes lots of support from multiple people and a lot of effort on their part too. Sometimes, unfortunately, hospitalization is for the better. I hated when my sister was in the hospital but I can't deny that it helped her.