Is it permissible to offer money to a friend who will marry a Kitābīyah (Christian or Jew), while fearing that the money will be exploited in a Western-style wedding party?

If you predominantly believe that your financial assistance to your friend will be spent on something forbidden, then it is not permissible for you to give him money, based on the Almighty's saying: "And do not cooperate in sin and aggression." Furthermore, assisting in good deeds and giving charity to those in need is more worthy and more beneficial for you.