Answer:
Stop Smoking
The difficult aspect of giving that advice to a smoker, is that everyone has already heard that smoking is bad for their health. Every cigarette package carries a clear warning label. So you would not be telling them anything that they don't already know. Smokers are people who know that smoking is bad for them, and who smoke anyway. Therefore in most cases I wouldn't bother to say anything. But if I really cared about someone, I would tell them that whatever benefit they think they get from smoking, they will eventually discover that the price is too high, and by then it will be too late.