I think if the younger pilot did believe the older one to be an instructor that would explain a lot...one sometimes sees a mentality among student pilots or even qualified PPLs that 'it will be OK since I have an instructor on board' and hence mentally they relieve themselves of some of the consideration for the flight and also of any worry they may have.
I agree 'command' decision making should be underlined more and the state of play should have been made clearer before they got into the aircraft. The situation where a new PPL is flying P1 but has a more experienced pilot next to him (but not an instructor) so they can 'take over' if things get difficult is not always ideal because it can lead to confusion over 'who is in charge'.