It's understandable that we don't want to offend or be attacked by someone for telling them they shouldn't be smoking when they don't want to hear it.
This is a way you can show your empathy without offending.
Simply say "I did that for 40 years " (fill in the blank for however many years you smoked)
That's all you have to say,
If they want to talk about it they will ask you how you stopped.
If they aren't interested, you lost nothing but, you have made them think about what they are doing in a non confrontational way..
And remember, you are not out to convince them of anything.
They are the ones who have to choose. All you are doing is planting a seed.