I hope you understand how serious this is. A young person wanting to end their lives, in my opinion is one of the saddest things in this world. They have so much to live for, and so much more to get out of life. What I am going to suggest you do is going to make them hate you. It is going to not make them your friend anymore, but you need to do it.
Tell the authorities, or more specifically tell an adult (preferably from school, such as a counselor) who will know exactly who to contact. Years down the road these people will thank you, but you need to act now. You cannot wait for this. The power of a group is stronger than the power of one. They will be more likely to go the entire way with it than if one of them was by themselves.
If you would like to talk more about this, please feel free to email me at spartan4eva@gmail.com.
~Shaun