I actually was faced with the sheilah once of whether I should return cigarettes to a smoker.
On the one hand:
They were in a place where the person would find them if I did not return them
He used to smoke in such a way that many people (including me) were forced to spend hours inhaling the second-hand smoke
He had other cigarettes at that moment
Lifnei iver?
On the other hand:
Hashavas aveidah
He would keep smoking no matter what I did
I believe (IIRC) that I was told to return them.
I read a great teshuva from the YU roshei yeshiva that would assur smoking. I don’t remember if it touched the lifnei iver aspect.
I know that my grandfather used to smoke and was told by R Moshe that bemetzius he should not smoke so he stopped.
Lung cancer is not pretty.
(I am aware that very little of what I just wrote is germane as far as the matter at hand is concerned- I just have very strong opinions on smoking. The week after a relative of mine passed away of lung cancer I literally yelled at a yeshiva guy who was smoking. SO yeah.
My friend had to actually drag me away.
And he was also wearing a Kahane Tzadak sticker so that didn’t really help matters.
Boy, everyone will know who I am now…)