Override Devise :: Mailer

Question

I am trying to add a method to Devise :: Mailer, the method must send mail to the user with a link in order to set his own password when the administrator creates a new user without a password.

Devise::Mailer.class_eval do
  def set_password_instructions(record)
    setup_mail(record, :set_password_instructions)
  end
end

If I put this in / lib and requested it in application.rb, it will boot up to the real Devise.
If I require it at the top of my user.rb, it works the first time, with another unknown set_password_instructions method at another time.

It only works when adding the above code directly to user#send_set_password_instructions

user.rb:

  def send_set_password_instructions
    Devise::Mailer.class_eval do
      def set_password_instructions(record)
        setup_mail(record, :set_password_instructions)
      end
    end

    generate_reset_password_token!
    ::Devise.mailer.set_password_instructions(self).deliver
  end

Could someone shed light on this strange behavior?
Looks like Devise.mailer is reloading the Devise :: Mailer class?