You are right to be worried. You are way over your head. Your friend is feeling worse and worse, and you are feeling more and more responsible for her. As much as you may care for her, you don’t have the skills and training that a person needs to effectively help someone who is this depressed. Furthermore, she is out of state and out of reach of any real help you could refer her to.
There are some confidences that you are not obligated to keep. Suicidality is one of them. It’s long past time for you to talk to your parents about talking to hers. Her parents need to know that your friend is seriously depressed and likely to harm herself. Yes, it will be difficult to tell. Your friend may have sworn you to secrecy and may never talk to you again. But think how much more difficult it will be if she hurts herself and her parents ask you why, oh why, didn’t you let them know so they could maybe do something to prevent it. Can you picture living with that?
Meanwhile, do give your friend the phone number for the National Suicide Prevention Lifeline: 1-800-273-TALK (8255). Trained counselors are available 24/7 to talk to her. It is absolutely free and confidential. For more information, check out their website at: http://www.suicidepreventionlifeline.org/
The counselors there can offer her far more help than you can. Please turn her care over to her parents and the professionals who can provide her with the practical support and advice she needs.
I wish you well.