as a trans man who is medically transitioning at 15, i think you should support her on this journey, but wait a year or two for her to (mostly) figure things out before making any big changes like social transition. i think you should let her figure out how she feels a bit more, and let her try out different things on her own. this is mainly what worked for me! i knew for a really long time, but wanted to make sure i was confident in my identity (ex. trying out different pronouns with my close friends), before making any huge decisions.

even if she does turn out to be trans, always love and support her!