The important thing that no one mentioned yet is to encourage her to go to church if she isn’t going to one already. Politely invite her to yours and give her some airspace to meet new people and form other discipling relationships as well. I would advise against taking on the full task yourself; let others assist you. (This does not mean that I do not have confidence in your spiritual abilities, I’m speaking out of the general principle that trying to do things alone spiritually is less effective than with other people of varying perspectives.)
Also, although it may be difficult for you, you may want to take this up with your youth leader or pastor as well so they can pray for you and assist you in this new task you’ve been taking on.
I agree that John is a good place to start.
Also, pray for her. And try to be kind to her as well, especially if she comes from an unbelieving family. You may not know what she is going through right now fully, and that is okay. God has a plan for your friends’ life and he graciously allowed you to be a part of it, so you can trust him with what you don’t know.
You probably know this already, but this is no place for moguls. Alright, it is time for me to stop talking, I’m probably chewing my toes. And you know more about this situation than me, so feel free to ignore my advice if it doesn’t make sense. Etc.