The first thing to do is tell her parents. I know it sounds like a betrayal but I assure you her parents will know how to help, even if they have to have her committed to a hospital. It sounds scary but its for the best, I would know I have been in her shoes. Going to the psychiatric ward was the best thing that ever happened to me! If I hadn't I know for a fact I would not be around today. Calling 911 is another option, they too will send her to the hospital where she can get the professional help she needs. You are not a doctor and cannot give her a will to live, but with time meds and or therapy she can get better! Best of Luck!
You need to call the numbers the guidance counselor gave you, you aren't a therapist and you are only 18. If your "friend" is actively suicidal call 911, that's all you can do. You are too young to be able to handle this.
I suffer from depression. I have never been in as rough shape as your friend is but I have been in trouble. This does not sound like something you can handle on your own. Its going to burn you out. If she is threatening to kill herself and has a plan, buying a gun, then something needs to be done soon by a trained professional. If she will not help herself call the police and tell them what she is up to.
Don't try and fix it on your own. You don't have the strength, no one does. Don;t figure her problems are your fault. If the worst happens it is not your fault.