Fidelis Mortuus, your self righteous friend is clearly ignorant about the health impact of smoking and it's very doubtful that facts will have much of an impact on such a person so the best thing to do is to point out that you'd rather not hear them sound off on the issue. If they continue to be in your face then that should be the end of the friendship.
If do wish to debate the matter it would be best to do so from a position of strength. What I suggest is actually read up on the matter and the best single resource is here: http://www.lcolby.com/
The bottom line is that the actual health risks of cigarette smoking is wildly overstated and a great deal of the research on the matter is grossly biased and heavily flawed. I could go into details on the matter but I am a bit pressed for time and I would rather not repeat myself over and over. Start reading the link above and get back to me.