The theory of flight is the same, BUT unlike in light singles (where the performance characteristics are very similar) in an airliner the speeds for take off / landing vary massively, as do pitch angles and cruise speeds. There are all sorts of handling characteristics which a PPL would not know. If there was someone to talk them through it, then it might be possible. It's not that a PPL is not sufficiently trained to fly an airliner (technique wise), but that they don't know how to.
The other problem (and probably an even bigger problem) would be how to operate the systems. My aircraft is fairly modern and "easy", but to do the absolute minimum to get it airborne you need to.
- Battery master ON
- Wait for system to run tests (1-2 min)
- APU switch RUN
- After APU has finished BITE tests, turn it to RUN and hold for 2 seconds
- Once APU has run up, system completes 15 seconds more tests
- Hydraulic pumps ON
- Engine run switches ON (one at a time, each start takes 1 min)
That'll get you enough to just about get into the air, but there's no navigation, no attitude indicator, no radios, no performance info, you'd need a huge runway (slat/flap not selected) Each of these items is a button or switch "somewhere" in the flight deck and has to be operated in the correct sequence. It's very simple when you know how, but without someone telling you how to do it and where the controls were it'd take hours of trial and error.