Firstly, I have no idea why the aircraft turned back and I am not speculating on the reason here.
Regarding the charts that should be on board, there should be two hard copies of the destination and alternate charts and one hard copy of the enroute charts for Europe.
The problem is that flight libraries change depending on where the aircraft is going. There are three possible options on the -400, Pacific charts only, Pacific plus Europe or Pacific plus Americas.
Last minute aircraft changes mean the correct charts may not be on board. Flight libraries are supposed to pick it up but these things do get missed occasionally.
F/Os (and S/Os are delegated the role by the FAM when they are carried) are the last line of defence. It's an antiquated and outdated system that has caused problems and significant expense in the past.
There is a strong argument for an EFB capable of carrying all required charts - Pacific, Europe and Americas - in one easy to carry format. Now, where's my iPad?