What should I do my best friend is possibly suicidal?

[Maggie]

I know it may feel like you're betraying a friend, but if you do think she is at risk, I encourage you to let someone know, and I encourage it greatly. That's far from betraying, believe me. By the sounds of it, she's standing on an edge, and what she may do is unpredictable. I think you should definitely report it, ASAP. To her parents, to a teacher, a friend, an adult at school, your parents, someone you trust to help her. She does need you to do this, though she denies it. And so would the other girl who seems to be going through the same thing. It's so much better safe than sorry. Do report what she's said to someone.