I've heard of cases before where a flight deck emergency has resulted in a call for a spare pilot, and invariably the spare is given the job of operating the radio and monitoring while the qualified pilot flies the aircraft. (OK, so UA232 was an exception.) She was flying into an unfamiliar airport, so having an extra pair of eyes on things to help with the workload seems an eminently sensible decision.
As for 'failing' his assessment, as an experienced military officer, I'd say his response would have been more to provide reassurance and calm to get the pilot into a better frame of mind for the task because that would have been a safer approach than trying to take over. That's what good military people do, build up the confidence of people on their team to achieve the goal.