A tenant organization or district attorney or the Attorney General bringing a successful action under this act, including where the action is ultimately discharged under section 25(k), shall be entitled to recover the cost of the suit, including reasonable attorney fees from the landlord, if the landlord had refused to initiate an action within ten days after having been requested to do so in writing, delivered personally or by certified mail, return receipt requested. If the court determines under section 9 that a landlord failed to prosecute an initiated action with reasonable diligence, the landlord shall be responsible for the payment of all reasonable costs of the suit expended by a prevailing substitute plaintiff designated under section 9.