How can i ask my friends to stop smoking without them getting mad?

Two of my friends smoke and I just figured it out today. I want to ask them to stop smoking, but I think that they will get mad. They started when they were 4 years old and their older sister tought them their parents already know, but they don't seem to care. My friend told me where they get their smokes from and I don't know what to do. Please,someone help me.