What to do
A way to help that person is to talk to them, take it seriously and follow it through. Ask them question like how would you do it and when? Have you ever tied it before? etc.
Make them feel important. Tell them that you care.
Listen to them, let them tell you how they feel. Don't interrupt them.
Show support. Tell them that you understand.
Don't go around telling other people. Keep it a secret except to an adult.
If they are feeling bad or they need to talk to someone help them.
Tell an adult.
If you think that person might attempt suicide in the future make a promise that, that person will call you first.