When a pax I've been asked before pushback by a (probably) overzealous flight attendant if I would be willing to assist in the event of an emergency (I have a baggage tag which indicates my line of work). My seat mate's eyes grew large as saucers as I tried to explain that it was very unlikely such a thing might happen.
When up front I've had a pilot in the opposite seat (not rated in category or class) on a familiarization flight which was diverted to a SAR case. While I could have handled the flight without him, he was definitely useful and I was glad of his assistance. Granted I knew the fellow, and we were part of the same organization so I knew his capabilities as well as his limitations in my machine.
Basics of monitoring flight parameters, copying clearances, and the like, are fully within the capacity of any multi-crew qualified pilot, whether that pilot is going from a B-1B to a 737 as in this case, or coming from a twin turbofan into a helicopter, as was the case with my colleague. If that pilot has operated a FMS, so much the better, but aviation radio tuning ain't exactly rocket science, whether it's a King stack or some other interface. My working ride at the moment has the same FMS as many current Boeing products. It just also happens to have rotor blades.
Things like seat adjustment are red herrings unless you intend the person to do any of the handling, which I suspect is unlikely. In any case, there is this procedure (perhaps you have heard of it) called "holding". A couple of requested turns might be prudent rather than trying to shove someone into the opposite seat inside the FAF.
Every case is going to be different. A non type rated pilot coming forward from the cabin may not have been useful for UAL 232 as a type rated training captain like Dennis Fitch was, but you never know. In the end, it was the FOs call to make, it turned out successfully, and only the truly churlish would second-guess that.