How do you talk your friend out of suicide?

It depends on who the someone is. If it is someone you care about, then stay with them, hold their hand and emphasize the relationship you have with them (friendship or otherwise). Make sure that you listen to them and give them the support they need. Give them a light in the distance to look forward to and most importantly don't cause anymore pain. Encourage them to visit sites that are aimed at preventing suicide and see a psychiatrist.