Praise be to Allah.
People should not pray behind a smoker if a better person can be found to lead the prayers. If no one better can be found, or you come and pray behind him and he is still a smoker, the prayer is still valid, in sha Allaah. The majority of scholars stated that prayer behind a faasiq (rebellious evildoer) is still valid, and if a person prays behind him he should not be told to repeat the prayer, because of the general meaning of the hadeeth: “Pray behind the one who says Laa ilaaha ill-Allaah.” (Narrated by Abu Na’eem, 1/320; al-Daaraqutni, 2/56).
And Allaah knows best.