If I have a friend who is a smoker(I still have a few left, no
pun intended. Most of my friends aren't anyway), I will keep
reminding the person of the negative aspects of smoking and the
consequences later in their lives. Hopefully the person will change
their mind.
Human's nature are that they will treasure certain things in life
only when they knew they are about to lose it 
This I'll do. I won't wait till their funeral then say:"As a
friend, I should have made a difference in your life and not let
you die from cancer(smoking-related). My regrets, may u rest
forever in peace. I'll do my best for your family"
I mean, why not let my friend take care of his family himself
