I’d ask her how she thinks other girls feel? Then reassure her that her feelings are totally normal. I hated puberty. I hated my breasts growing. I was hugely angry at the changes. I hated the extra hair, I hated the fact that I had become different and changed and I didn’t want to. I felt angry, self-conscious and upset. I felt I wasn’t me.
But a few years later, those extreme feelings eased. Understanding that others feel the same, that’s it normal and natural, should help her. Also, try to help her feel good about herself, not just how she looks, but what’s she achieves, her hopes for the future. Be extremely careful about casual comments. I took the tiniest thing to heart and was very sensitive.
I’d also subtly try to make her feel proud to be female. Casually and carefully talk about the silly outdated ideas of the past, and how great it is that women are now scientists, sports stars, explorers, astronauts, etc, etc.
Let her wear baggy clothes if she must (my niece is very into baggy clothes as she develops) but tell her that binders will damage her breast tissue and her body, and that she shouldn’t feel ashamed of her body, that female bodies aren’t things to be crushed or deformed. She is unique in all the world - every part of her, and she will gradually realise this and be proud.
Watch out for friends telling her crap and be clear gender identity is a matter of belief not fact.