Well... actually, the truth is, it's the smoker's choice to either continue to smoke or quit. If we can't stop him/her from smoking, there's not really much that we CAN do. But I'm not saying that we should just watch them destroy their lives like that. I've watched something in the TV that states why people smoke and how to stop them from smoking.
It turns out that when you you smoke, some kind of chemical enters your brain and triggers an area of it that controls pleasure. I don't really remember the details, but I guess that's why people get addicted to smoking.
But of course, if a person you really care about is smoking, you should try these 3 steps:
(1) Show them that you care. Yeah, that's right. Don't scold them or anything.
(2) Tell them that it's dangerous to their health. I know many commercial break say that, but they should really hear it from someone who cares.
(3) Always be there to help them. I can't say much on this, LOL.
Anyway, I just got that from the TV show itself, but I believe that those three steps can really work.