A good doctor is well aware of what anorexia and other eating disorders can do to a girl and will therefore not say anything to a teen or young adult about her weight, unless the girl is grossly obese. A better tactic, if the doctor is concerned about a girl’s weight, would be to ask the mother privately about what she serves her children and make appropriate suggestions based on that information.
Regardless, it is better for girls to have female doctors wherever possible.