There is no easy answer.
Personally, if she is sweet and wants to bring along her friend, I would have no problem. I might ask her to be discrete if there are kids nearby, but tell you the truth with all the stuff on tv (e.g. Glee) even here in the Philippines my 10 year old granddaughter knows about gays.
If she asks if you approve of her lifestyle, well, answer it the same way you would anyone else who is divorced, living out of wedlock, drunk, cheating on the spouse, a member of the mob, or a republican politician running for president(;->):
Tell her you love her but you are praying that she will find God’s love in her life and grace to follow his commandments.
Kids should be protected from TV shows that promote the lie that "gay is good". If the child didn't see that stuff she would not be influenced, better kids know about such things when they are older and have learned more about right and wrong.