To try and answer what you're asking genuinely, since I don't see indication that this is bad-faith:
The reason African-Americans and some black people get a "pass" to use it isn't because it isn't offensive. Its because they're expected to have a better understanding of their own oppression than someone who hasn't lived with it, and because it can generally be assumed to not have racist connotations. The word has such heavy history that if someone who isn't black uses it, it is usually (and reasonably) assumed to be racism. Even among those who are black, theres a lot of nuance, with many feeling uncomfortable with saying it, either because the word is so loaded that it can be offensive reguardless, or because they don't feel like they're connected enough to the history to have the "right" to say it (I.E. someone who grew up in a black-majority country might feel this way.)
Be respectful but honest. Hopefully you can still be there to support her, but its not your responsibly to hide your sexuality for her, nor would it be right to lead her on when the relationship won't be successful.
Just try to keep in mind that she's likely in a vulnerable place right now, so while you shouldn't lie, try to be gentle. Prehaps emphasize that theres no blame, and make sure she knows you're proud of her.