It’s really common in girls this age. The teenage years are tough - periods are inconvenient and often uncomfortable, breasts are embarrassing, she will have been exposed to sexual “banter” and sexual harassment and will see herself being funnelled down a path to objectification and unrealistic expectation that she wants to jump off. Somehow it doesn’t matter how many “normal” or diverse girls and women that girls know and see irl - the media content sticks and the message is overwhelmingly that girls and young women are sexy support animals and older women are ignorant, boring, Karens. Who wants that? “In my day” opting out of your sex was ridiculous and unthought of whereas young people today are having it presented as a completely legitimate solution to patriarchy.
Obviously sex is binary - you can’t be a non binary sex, although DSD conditions have been hijacked to pretend that nobody knows what sex they actually are (but they do know that some people are really the opposite one) and mermaids and others encourage removing or hobbling sex characteristics - their ex CEO famously blocked the puberty of her own son with hormones sourced from the USA and then flew him to Thailand at 15 to have his penis and testicles surgically removed on his 16th birthday. They ghoulishly sang “happy birthday” in the operating theatre. This (the castration, not the singing), lead to a change in the law in Thailand but is still the “gold standard” of “trans youth healthcare” advocated by a section of gender identity advocates.
If she understands that her sex is binary and she is female then what is it that she is non binary about? Gender? Explore what that is? Is it an internal sense of self? In what way is it tied with names, pronouns or sexed bodies? How do you “feel” a different sex? What is that feeling? Can you feel a different race? A different age? Is it anymore than not conforming to your own rather limited expectations of what people of your sex can do, be, or feel? What is it she thinks she can’t do as “girl” or “female” that she can do as “non binary”? Does she think other women and girls should not demonstrate the behaviour that isn’t conforming (whatever that is - be specific about what she thinks - is it a performance of femininity? Do ALL the women and girls she knows do this performance? Is it wearing particular outfits? Haircuts? Hobbies? Jobs? Childcare and domestic labour? - what women does she know who don’t conform to whatever stereotypes of women she has - are these women “non binary” or “trans-masc” or “trans” or are they just having their own personality plus a female body? Can you play rugby as a girl? Drink pints? Work as an engineer? Fart? Have a cheap, short, natural coloured haircut? Have short nails? Wear trousers? Have male friends? Tinker with cars? Say “I’ve emptied the dishwasher for you”? Specifically what is it that she is non binary about that other women and girls are “binary”?
I don’t see gender as an identity or anything internal at all. Gender is a hierarchy that positions males at the top (the natural leaders - logical, sensible, strong and clever) and females at the bottom (emotional, can’t drive, stupid, the service class) so naturally I want to opt out of it - but that can’t be done as an individual - you can’t just say “I’m not like those other girls - they are stupid , over-emotional, hair obsessed sex objects who need to be in the kitchen making the sandwiches, I’m an actual human”. She’s just as female as the rest of us and society doesn’t just decide to oppress those who opt in to their oppression. Does she think the girls who identify as girls should be subjected to the negative consequences of femaleness that she wants to avoid in an individual level? If you can identify out of oppression, then the logic extension is you can identify in - it’s the fault of the oppressed, not the oppressor. One of my biggest beefs with non-binary identities is just how rude (and privileged) this stance is.
Same sex attraction is an additional complicating factor often heightens at this age - even amongst girls who will ultimately grow up to have exclusively heterosexual relationships - both sexes often “practice” sexual attraction on their own sex as it has a safe familiarity - girl crushes are entirely normal (and likely) and she might actually be bisexual or lesbian and confused about this in terms of “gender” and sex. For some people being a straight boy is preferable to being a lesbian girl.
My advice - don’t “socially transition” - definitely don’t medically transition, challenge the misogyny and conformity of it (the “girls think/do/like X and I don’t therefore I’m not a girl”) and let her know how loved she is for her unique personality in her perfect body that belongs to her. Actively demonstrate how other women are achieving things and having fun in their female bodies. Point out the legions of women globally who manage to do exactly what they want despite being “binary”.