It isn't going from being underweight to being fat... the middle portion of the spectrum (appearing healthy, at a normal BMI) is missing.

Perhaps what applies as a "normal" weight for an anorectic is to the anorectic "fat". The goalposts are different, not just because the self perception is different, because the expectation is different. It's a whole different ballpark. Perhaps that is not as Dichotomous as you would think, it's just seeing a different spectrum of grey.

I would suggest that perhaps putting yourself between someone suffering from an eating disorder, and their eating disorder will only widen the gap in your relationship. Anorexia is unlikely to be an enemy to the sufferer, it is their ally, friend, and abusive partner. It may be the world that is the real enemy.

Perhaps it is more helpful to find out a Before Anorexia and an After Anorexia perspective. Why is being underweight so much safer than being normal? Why is normal so frightening?