When I originally thought about writing this, I realized that everything I had to say has already been said--that eating disorders (EDs) are often more psychological than people realize, and can often be more about control than maintaining a certain body image--so instead I thought I'd write a little bit about how to intervene (and how not to) when someone you know has an eating disorder.
As the article linked above mentions, it's important to realize that EDs can come in a variety of forms, and may not necessarily be classified as anorexia or bulimia. It could even be as simple as dieting, which is at times hard to distinguish from a healthy concern for getting proper nutrition.
Whatever the case may be, there are a couple general things to keep in mind when trying to help somebody-however good the intentions may be.
First of all, don't make the person feel like there is something wrong with them-EDs are way more common than people realize, and they can be caused by a number of factors.
Get to the root of the problem. Like I mentioned before, EDs can be triggered for a number of reasons, and in order to get out of it, you have to get to the core of what's wrong. This of course could take awhile, but you could try talking to the person and figure out how they are doing overall-are they going through a particularly rough time? It may be directly related to body image, or it may be about gaining control after something traumatic happens. Whatever the problem is, talking through it may be really helpful.
Don't be forceful. Although it may seem easy to just force someone to have food, or a certain type of food to get them to gain weight, this will probably just cause a backlash and make things worse.
If the problem is Body Dysmorphic Disorder (BDD), it may be helpful to get someone to see what they actually look like. It's really useful to be able to compare to someone else-numbers (ie measurements, clothing sizes) would help!
Lastly, make sure the person knows that they are not alone-let them know how common it is. This is probably best to do after the problem has been acknowledged, and the person is willing to get help.
I don't mean for this to be a comprehensive list of all things you could do to help someone with an ED. Each situation is different, and the severity can really vary. This could go on for years, or or it could be short-lived, or it could reoccur throughout someone's life. The important thing to remember is to work with the person, and figure out exactly what it is they need.