If your child is in denial of his thinness, you must not be so. To say nothing is to delay the care. But the earlier it is, the more likely it is to lead to a cure.
The leanness of your teen worries you? React! Tell him that it worries you, that it would be better if it takes a little weight to be prettier ... The mere fact that you tell him your feelings can stop the disease if it is only at its beginnings. Above all, do not insist. Similarly, do not force her to eat the contents of her plate. But explain to him that all foods are good for the body and that banning certain categories of them like fats or sugars is dangerous because the human body needs them to function.
In addition, try to understand what drives her to starve. Offer him your help or at least your listening. If the flow goes wrong, a brother or a friend may try to ask him why she inflicts such a torment.
Offer him also an appointment with a psychologist specializing in eating disorders. If she refuses, contact associations. Some of them offer hotlines so that teenage girls can find the help and listening they need. On the other side of the phone are psychologists or former anorexics who will find the right words to make your child understand the importance of healing.
Diabulimia: The World's Most Dangerous Eating Disorder (September 2020)