How can I convince a friend that smoking cigarettes, even though it is forbidden, does not expel a Muslim from the fold of Islam, nor does it make him an infidel deserving of hellfire?

Your friend erred in excommunicating you (takfir). It is dangerous to definitively declare someone destined for Hellfire when they outwardly profess Islam. Not every sin is an act of disbelief (kufr), and declaring someone a disbeliever based on every sin is the doctrine of the Khawarij. Explain to him that even if someone commits an act of disbelief, they are not ruled to be a disbeliever unless all the conditions are met and all impediments are absent. How then can one declare disbelief for a sin that is less than disbelief?