A girl asked me if I have feelings for her, I said no-she got mad and asked to have sex with my best friend. I said ok while they were having sex I realized my love for her. Do you think I was to late in this all?

From what you shared, it sounds like this girl is struggling with self-esteem issues wherein she seeks validation through sexual attention. Given that she is acting out sexually, I do not recommend getting involved with this girl, as her motivations related to sexual relationships do not appear to be about connecting with another person, but rather about using others as a temporary "fix" to get her self-esteem "high." I think it is important to think of her as an addict, because that will remind you of the gravity of this situation and help keep you away from this girl.