If I buy something forbidden for someone, what is the ruling on that?

It is not permissible to buy a forbidden item for another person, as this constitutes aiding in sin, in accordance with the Almighty's saying: "And cooperate in righteousness and piety, but do not cooperate in sin and aggression." Sheikh Ibn Baz issued a fatwa stating that it is not permissible to assist a father in buying cigarettes, as this is considered aiding in disobedience.