I recently gave a shiur on lifnei evar and i came across a Netziv on Lifnei Evar which explains that there is no problem for a jewish butcher to sell non kosher meat to a non observant jew because the isur is not taking place right then. If one holds that smoking is asur, it would seem that there is no problem with selling cigarettes (provided that one can buy the cigarettes from someone else) since the isur is not taking place at that time. Would there be an issue with offering someone a lighter to light his cigarette? Please provide sources. Thanks!
Rav Moshe Feinstein (Yoreh De’ah 2:49) writes that because smoking is an accepted practice among some people (and some Torah scholars), there won’t be a prohibition of lifnei iver in giving somebody a lighter even for those who consider it forbidden.
He clearly assumed that if we consider smoking absolutely prohibited (over time, less and less people smoke, and the rationale noted by Rav Moshe becomes weaker), lifnei iver will apply.
Note also that the Netziv you mention (which we have cited in our article on the subject, see here) applies to the prohibition of mesaye’a and not to lifnei iver.