I agree with Chris above. The B757/767 is a user friendly aeroplane for experienced pilots to operate.
I reckon you could teach a competent multi-engine IFR pilot to fly one to a basic airline First Officer standard in a few months.
However a novice pilot would just be overwhelmed if he or she could get in one and even if someone could get the engines started and manage to taxi it, everything would happen so fast on takeoff and initial climb that a bad outcome would be highly likely.
A successful manual landing would also be highly unlikely. The human brain can easily be overloaded if ‘thrown in the deep end’.
If the fictional character could get some instruction in a real airline simulator the scenario would be more plausible.
A private pilot or advanced student with some simulator training could plausibly learn how to program the FMC and get a B767 into the air, get the autopilot on and fly across the Atlantic.
The aircraft will do a full automatic landing if the airport has an ILS and is set up and operated correctly, but it would take at least a few days of simulator training to be able to do a complete flight.
Using a Microsoft flight simulator with a B767 add-on, and there is a good one, could be used to learn some of what would be needed.
The desktop simulation is good enough to learn how to get the a/c going from ‘cold and dark’, program the FMC, use the autopilot and quite a bit more but translating that to the real aeroplane for a complete real flight would be a big stretch. Implausable IMHO.
Also the good B767 add on for the MS flight sim was not available until about 2000.
BTW, it is plausible to me that the 9-11 murderers chose the 767 and 757 because that add-on allowed them to learn the FMC, autopilot and glareshield Mode Control Panel. But their plans did not include taxiing, takeoff, cleanup, approach and landing.
I would not like my family to be in the back of a jet that I had no experience of operating if I was forced at gun point to do it by myself, and I have 22 years of jet airliner experience.
A cousin of mine who was an Air Force maintenance engineer got his father into a B707 simulator at least a few times in the '90s. (Not in the USA). They flew it for hours, with motion off. If they could do it, so could your fictional character. You don't need the hydraulic operated motion on to run the software and have a pretty realistic trainer.