The early phases of an eating disorder may be praised and reinforced by family and friends because the initial symptoms might appear to be positive. Weight loss, for example, is often praised, along with behaviors such as "healthy" eating, exercising, and even other achievements in school, sport or other activities. For the person prone to anorexia it is likely that s/he has some traits of perfectionism.
As these achievements continue to get reinforced by others the individual continues to set even higher expectations for him/herself and perpetually fears failure. The problem is that nobody is perfect so it becomes a never-ending battle. No weight is low enough, no size is small enough, no test score is high enough, no amount of exercise is long/hard enough. The eating disorder thrives on this.
For a caregiver in the life of someone with an eating disorder it's important to recognize the ways that you are giving positive reinforcement to the behaviors and stop. No more comments on weight or appearance. Not even, "You look nice today." Because guess what, the ED warps that to mean that s/he didn't look nice yesterday or that you just said s/he is fat. (I know it sounds preposterous, but that's really the way appearance-related comments get translated).
Find ways to connect with your loved one without praise. See a movie, visit a museum, go to the dog park, do a puzzle – anything that isn't achievement-based. Keep the conversation on topics other than food, exercise, weight, health or anything related to the ED. It's so important to show him/her that you love who s/he really is as a human being, regardless of achievements.