As society grew more aware of eating disorders, those of us who were fat or very thin began to hear that we MUST have an eating disorder. Meanwhile the average-sized person with an eating disorder went unnoticed. It's not our place to diagnose other people.
For those of you who are genuinely concerned about a friend, there are lists of observed behaviors you might consult but don't base your talk on their body size. Not if you want to keep them as a friend.
Why You Can’t Tell If Someone Has An Eating Disorder Just By Looking At Them See also
Lindy West on How Eating as a Woman Is Like Being Under “Surveillance” and
http://everydayfeminism.com/2015/08/size-discrimination-facts/