Okay, I'm a Christian and I don't believe homosexuality is right. It specifically forbids it in the Bible, and I think that it is a sin, and I would never kiss another girl.
BUT. If another person is gay or bi, I don't judge them for it. That's what being a Christian is all about. I wouldn't necessarily support them, or agree with them, but I wouldn't treat them any differently. I would probably pray for them a lot, but thats not a negative thing. I don't condemn gay people, because if that's what they feel, then they can't help it, especially if they are not religious. So I wouldn't judge them for it.
I would tell your friend, and remind her (nicely) that being a Christian is NOT about judging people. If she is a born again christian, she should accept that and still be just as good a friend to you as she was before.