Do you think my friend is a lesbian?

If she said ''i cant believe she's saying that about us'' when your friend was making fun of lesbians, it doesn't just sound as though she's a lesbian, it sounds as though she's somehow come to the conclusion that you are both in a lesbian relationship. If it's creeping you out you should make it obvious that you want her to stop, but at the same time you should let her know that it's ok for her to be openly gay/bi instead of her just making really obvious hints and unwanted advances on you...