So, shouldn't this be simple for me, as I've already come out to everyone? I don't know how out she is, so I'm not telling people. I just didn't know what to say, other than that (obviously), I don't judge people based on sexual orientation, gender identity, etc. One thing though, to know that somebody trusted me this much felt really good. I don't know if I should say more about it to her. Any advice?
That's actually great that a friend of yours come out to you. I know for a lot of people, a LGBT friend is the first person that they come out to. I'd just let her know if she needs any help with things like acceptance, coming out, etc, that she's free to talk to you about that.
Make sure to let her know that you have her back. Its nice that she trusts you with something so important.