Anorexia Nervosa is a notoriously difficult condition to treat, with a multitude of reasons for its existence. Strictly a case for the professionals. Psychiatrists, psychologists and CPNs have battled with this condition for many, many years.
Paradoxically, any attempt at emotional support may reinforce her belief in her own way of thinking. The only real cure is large doses of Anno Domini for these troubled youngsters.
Totally agree with Scylax. Do not even try.
The reasons for anorexia are very complex and there is little that anyone can do. Professionals really struggle with these youngsters, but fortunately most of them get through it in the end.
Sorry to be so negative, but , you will find that these answers are as nothing in their negativity to the anorexic young lady.