Which eating disorder? I’ll give you anorexia as an example
Dear Friend,
I am worried about you. You seem unreasonably thin and exercise excessively. I just want to tell you that you are perfect the way you are, you do not need to change. The disorder I am worried you have is anorexia nervosa. A very harmful and addictive mental disorder. There are many causes I would like you to recognize. These include, trama, and need for control along with others. The health concerns are apparent. Osteoporosis and malnutrition are among the list. There are many places to get help. Including inpatient facilities.
Your Friend,
(Your name)