I saved the article below from a mental health web site–these are not my words.
What should you do if you think someone is suicidal?
Get help. The best thing you can do for someone who is suicidal is to get them some sort of help. You could get them to agree to go in for therapy and/or some sort of psychiatric intervention. If the person refuses to change and you suspect that they may take their own life, do not hesitate to call the police. Many people are afraid to call the police when a person is suicidal, yet it can be the exact intervention needed to turn a person’s situation around.
Prior to calling the police though, talk to the person by speaking up. Don’t argue with the person, just be empathetic to their situation and promise that you’re going to get some sort of help. Once you ask a few questions about their situation, determine the degree to which you think the person will carry out the act. Ask them whether they have a plan, whether they have materials, if they know when they would do it, or if they still have the intention.
If the person says that they have a plan and materials, you may want to recruit extra help. If you are able to remove potentially lethal objects from the person’s possession, take this step. Continue to offer the person help and support and encourage them to seek treatment. Also come up with a safety plan or contract to further minimize their risk of self-harm. It takes a lot of courage to intervene when someone is suicidal, but at the end of the day, you may save someone’s life.