The problem is that this passenger could be a clever, lying piece of s...t, trained on MS Flight Simulator, looking for an adrenalin kick, 15 minutes of fame and a story to lighten up his miserable life.
In best case.
He could also have been suicidal poor bastard, with revoked license longing to end his life in an epic pilotīs way, or why not high as a kite from sniffing cocaine as dessert, and eager as hell to show how to master a 744.
Or whatever else you can come up with.
Thatīs paranoia.
Let us look it from the cbin side. You are a qualified pilot (like in this case it seems to be) and are sitting in the cabin and being made aware of a problem in the pointy end. You offer your help and the remaining SOP slave rejects it. How would you feel, and how would other passengers, who sure get some information as well would feel for the remainder of the flight?
Even if he is not qualified on type he but a rated pilot on another type he will read a checklist better than a not trained cabin crew member, will be able to monitor communication or even be able to take over communication.
Are we that far degenerated, that a captains decision is not respected and questioned to death?