The most important thing you can do is to talk first to God about your friend. Only the Holy Spirit can convict her of her sins, and only the Holy Spirit can convince her of the truth of the Gospel. Before we ever talk with a person about Christ, we should first talk with Christ about that person.
What can you say to her? First, I suggest you commend her for being honest about herself. Yes, she’s on the wrong road in life, but at least she’s finally honest enough to admit which road she’s on. Then encourage her to have the same honesty about the life she’s living, and the consequences she’ll face if she continues with it. Right now, she wants to run her own life, but where will that lead her? The Bible warns, “The way of the unfaithful leads to their destruction” (Proverbs 13:15).
Then ask God to help your friend see Christ in you. She may argue with your words, but she can’t argue with your life, as you demonstrate Christ’s patience and love and peace. Eventually she’ll realize (if she’s honest) that you have something she doesn’t have—and that “something” is Christ.
In addition, ask God to help you know when to speak with her about your faith in Christ. She needs to know that God loves her, and He wants what is best for her. Thank God for the opportunity He is giving you to share His love with her.