This fanciful sort of speculation has been going on for years; I remember having a discussion on the possibilities of a passenger landing a Lockheed Lodestar - an aircraft several orders of magnitude simpler than a 747 - and concluding that a satisfactory return to earth would be unlikely.
The "landing" is never fully explained. Is this just a touchdown from short finals or a descent to altitude then further descent to intercept a localiser and all that follows? In any event, it is all academic.
First of all, the PPL has to get into the seat. This may sound silly but whilst the Seven Four is a very large aircraft, its cockpit isn't - at least, not widthwise and operating the mechanics just to get seated would be the first hurdle.
This Walter Mitty-type PPL probably has a few hours in something like a C172 or PA28 and is accustomed to almost instantaneous response from the flight controls. I submit that such a person would be lucky indeed (without the benefit of a few hours practice) to achieve something as straightforward as maintaining height and direction in a 747 where control response is vastly different, particularly in amplitude and timing and I suspect the aircraft is going to get away from our hero who is going to fall behind the curve very quickly.
Even with autopilot, RNAV, VNAV, CAT 111, and all the bells and whistles as well as cockpit assistance from another PPL and ground talk-down he would never manage it. "Speed brakes armed". Speed brake? If you don't agree, just consider height perception for starters. Normally his backside is three or four feet from the ground (when in the cockpit that is!); in the 747 he is on the fourth floor so rounding out for the flair is going to be done at minus fifty feet . . . OK, so someone is feeding him the radio altimeter but he just is not going to do it; something he realises quite early on but he has to stick with it because the only option is a go-around which will get him into even deeper trouble as he has to undo most of which he has just done and in pretty short order. And then there's turbine engine management . . . And if it's a glass cockpit . . . need I go on?
But it's still good Boys' Own stuff!