Well, biggest issue here is.. two different flights... to different crews, right? If Pilot A (and his colleague pilot C) from flight A broke the rules and invited a pax into the cockpit, it was his (their) decision. Pilot B on flight B might want to stick to ze rulez and don't risk their job. Why do you assume pilot B is lying? He just said, "sorry... regulation says I cannot take a pax into the cockpit, if I do I might risk my job." Which is perfectly honest.

It would only be a white lie if the flight crew and cabin crew on both flights were exactly the same. just one other crew member could trip it over. Assuming Pilot A flew back with Pilot D instead of Pilot C he flew in with... and knew Pilot C would never accept a pax in the cockpit... for sure pilot A would fall back to regulation. Same for the purser of FAs in F. If one of them reports the breach, pilot A would be in trouble.