She started out as bulimic, which eventually evolved into anorexia nervosa. She was average weight to begin with, but even with calorie restriction and purging (she didn't binge. She would purge after a normal meal with her parents) she didn't lose much weight other than around 10-15 pounds of body fat in the very beginning. And purging "doesn't actually get rid of the calories"? Do you know how the human body works? If you remove the food before it can be digested, your body will gain none of its nutritional value. The only exception I can see to this is heavily-processed foods made of bleached white flour and sugar. Even then, the food is not broken down within seconds. I'd estimate 15-30 minutes, which is well within the time most people purge after eating.
I'm not a nutritionist, and I think it's safe to say you aren't either. I, on the other hand, am not going to question the judgement and the knowledge of someone who deals with this kind of thing for a living. If nutritionists and medical associations consider it to be something to take into consideration when choosing a weight-loss diet, I think it's safe to assume that they have a reason for assuming that. If you look up "calorie restriction and still gaining weight", there are hundreds if not THOUSANDS of people who are confused as to why they are gaining weight even with calorie restriction. While it's safe to assume that several of these people are just compulsive eaters and aren't actually restricting their calories, the sheer magnitude of people who struggle with this SCREAMS causation, not just correlation.