i know lesbians in my all girls school, they dont crush on every single person you know, they have their own grilfriends and crazily they still have straight best friends who are supporting them
you say she could've said earlier, well people dont really feel they are homosexual till later in their life, so its a bit selfish
if youre not homophobic, then you should accept her as your best friend, she knows you're straight, doubt she would try to hit on you, and dating girls is not different to dating boys, she'll still give you advice...