Ha, all the spotters and enthusiasts and aerosexuals who comprise 90% of the forum these days must be absolutely wetting themselves with excitement reading this, they all dream about exactly this scenario.
Personally speaking, unless the guy was rated and experienced and could verify such by producing a licence or company ID, I'd far rather single crew it, or get a cabin crew up to read the checklists, it's hard enough flying with a type rated min hours cadet, let alone a want-to-be-helpful pax. There's just way too many risks and unknowns with letting someone in the flight deck who might or might not be what they claim. Would be way much stress watching them like a hawk to make sure they didn't push any buttons they weren't supposed to. Far easier to follow the SOP, call the mayday and do the single crew divert