Answer: Bulimia nervosa.
Step-by-step explanation:
She is most likely suffering from bulimia nervosa which is an eating disorder characterised by binge eating, followed by feelings of guilt. The resulting behavior is self induced vomiting. The decaying tooth enamel is a result of frequent vomiting which brings up acid from the stomach into the mouth and wears away tooth ebamel. Seeing that she exhibits this kind of behavior despite having normal body weight may be a sign of deeper underlying issues related to self image and/or depression.