Taubes said something like, It's not about the calories you put in your body, it's about what your body DOES with the calories you put in your body.

If we played semantics we could ALWAYS say that it's just a calorie equation, and if someone stays 200# while eating 500 calories a day (or me, who easily has stayed nearly 400# while eating 1000 calories a day), that it's "merely that their metabolism has slowed down so they need to reduce their calories to match their metabolism."

The problem is, this sounds great on paper, but fails abysmally in the real world. Plenty of animal and human studies, as Taubes book lists, show that insufficient calories will leave a person or animal FAT, while eating up their muscle and vital organs for energy.

So it seems sort of ... oh I dunno, semantically not-inaccurate, but a little patronizing as a real-world recommendation, to suggest that it's all about calories, since in many cases, UNTIL the "nutrient ratio" is adjusted to drop insulin so finally the fat cells will release it for energy and quit stuffing everything someone eats into them (no matter how little it is), weight loss -- unless you don't mind sacrificing muscle and vital organs for it (while maintaining the fat) -- is just not going to happen.

(There are also BTW a few studies that make clear that larger calorie numbers actually resulted in more weight loss than lower (when the eating plan was essentially highfat/lowcarb).)

So if her friend is eating 500-900 calories a day and she is still fat and not losing the fat, it isn't about calories; she is probably already eating up muscle every day, and if she dropped her calories any further to win that "calories in vs. out contest" she would merely be eating liver, heart and brain tissue that much more quickly.... not losing more fat.

The whole calories concept is based on assumptions about the laws of thermodynamics that are not accurate -- read Taubes's book, which is a far better explanation, for how that all fits together.