Many smokers already have a fear, whether it's expressed or not, that they can't stop smoking, or that they don't even want to quit. Others believe they can stop at any time but find out they really can't when they try. Some smokers feel certain that they'll suffer from some kind of smoking-related illness, and even those smokers who think they won't get sick have likely considered illness at some point. Being a smoker can already carry these fears, so constantly talking about things like lung disease, cancer and emphysema might be counterproductive.
Using fear to keep children from smoking often includes public service announcements with graphic photos of diseased lungs or assembly speakers talking about painful cancer treatments or shortened life spans. By adulthood most people have seen countless images and heard stories of the health risks of smoking, yet they still smoke. Lectures or well-researched presentations on the harms of smoking also can raise some ire. Smokers most likely are very aware of the dangers of smoking, and spouting facts can come off as being condescending.
If you want to take the fear approach, sharing your own fears of losing the person you love may be more effective.