My brother in law who is 17, asked me to buy him cigarettes in duty free in America. I said no because the damage of cigarettes is well known and since he could only get them if I bought them, that constitutes lifnei iver. He said he could ask someone else which I said I’m fine with. Did I do the right thing or was it permitted to buy him cigarettes?
You did the right thing, even if he would be over 18 years old. By purchasing the cigarettes for him you are helping him do something that is dangerous for him. Even if he can get it from someone else, and it wouldn’t be lifnei iver m’dorayso, it would still be misayeiya, which is assur m’drabonon.