OP here. I was hoping that someone with anorexia would write an answer, since this hasn't been the case so far, I decided to write an answer based on the external source of someone who had anorexia.
I'm writing this answer based on this external source. It's a blog post written by someone who had an eating disorder.
Here is a summary of what is said in this external source:
Don't comment on the person's body shape.
There really is no right or safe way to do it. Anything you say can and will be misinterpreted.
Especially, don't say:
But you’re already so skinny!
orIf the person wants to be skinny, then they will be happy about this and it will encourage them to become even more skinny.
On the other hand, if the person is trying to recover from anorexia, they might associate "skinny" with "ill" and this might discourage them.
Also, don't say:
You don’t look like you have an eating disorder.
orPeople who are overweight (according to society standards) can still have an eating disorder. And if people around them don't believe that, then it could be very dangerous for their health, leading them to not be treated for a disorder they do have.
Don't say either:
I wish I looked like you
orPeople with an eating disorder suffer and are constantly fighting against their brain. Telling them "I wish I looked like you" without thinking about all the pain they go through is very insensitive.
You also shouldn't say:
You look so healthy!
More explanations here will come later, but you could still read the blog to know about it.
However, here are some of the things you can do:
Take the focus off of their body. Some of the most thoughtful, kindest, unforgettable compliments I have ever received had nothing to do with my body image.
Here are some compliments examples that the person writing the blog suggest you to use:
Compliment on "their refreshing personality"
"their witty sense of humor"
"their exceptional intelligence"
"the sparkle in their eyes"
Some other relevant links:
10 Helpful Things to Say to Someone With an Eating Disorder - This link will help you know what to say to make the other person feel better about themselves, which, in turn, will help them heal from anorexia.
Understanding Anorexia -- the Basics - This will help you understand this disorder better, it will also give you some keys to prevent someone from becoming anorexic and tell you how to treat anorexia.
5 Reasons Why You Should Stop Complimenting People on Their Weight Loss - This link will help you understand why complimenting someone on their weight loss might not be a good idea.