something_controller.rb

class SomethingsController < ActionController::Base
  def create
    Time.zone = current_user.time_zone
    @another_user = current_user.friends.find(params[:friend_id])

    FriendMailer.message(@another_user).deliver

    # Wouldn't Time.zone be reset by the mailer before rendering the page here?
  end
end

something_mailer.rb

class FriendMailer < ActionMailer::Base
  def message(friend)
    Time.zone = friend.time_zone
    mail(to: friend.email, subject: "hello friend!")
  end
end