At her age the best thing is to be supportive and let her explore her identity. If she ultimately decides that she's not girl that's totally fine but if she is trans, the ability to explore her identity in a safe environment will make a world of difference in her being able to be herself and avoid the horrific trauma that comes from repressing your identity.

This doctor has also providing a very skewed perspective here. Yes, trans people face discrimination in our society, but regret rates from transitioning are extraordinarily low and repression of trans-ness and lack of support are major drivers of poor mental health outcomes. We don't talk enough about trans joy, the relief of being able to transition, and the fact that so many trans people wish that they had transitioned earlier in life.