Am I missing something here? Surely the main purpose of the standby horizon is to enable a defective ADI to be identified. The captain lost his instruments. The first check is to compare the FOs ADI with the standby. If they agree, the FO has control. I've been out of flying for a while now, but I believe the two systems are totally separate, meaning that if the capt's flight computers go down, the FO still has a full set. He didn't trust his instruments (?) or perhaps he didn't want to fly the thing (?)