ya know if he is a good friend, there is nothing wrong with a casual non pressing mention of your concern about it, when you are talking anyway. telling him you do not want to harp on him but are so concerned and do not want to lose him to this cancer and fear you will if he doesn't stop smoking ... as an ex smoker i would not have been offended or put off by that ,... it is not a guarantee he will stop because of it, but it would not be offensive and would help you feel better most likely as I can tell you are truly concerned (with good reason).

sometimes being a friend includes helping someone face reality and being accountable for their actions or in-actions ... we have to decide for ourselves if we feel close enough to the person to require them to be accountable ... i like that my real friends are there to call me on the carpet if i mess up... but some people get real offended by this (usually because / when it is someone they are not close enough to for them to accept it but sometimes it is because they are just not open to seeing their own faults)

but if your gutt is telling you to talk to him, i would say then you should.