Complete bollocks. Unless he lacks a visa or something there is no reason to stay in the transit area. Before I moved to HK, I did a transit through HK and left to go to see the city. Many people do so.
EDIT - where is he actually flying to? I am wondering if this is one of those daft new rules for flying to the USA - but it's a long shot. I would imagine it's more the Japanese airlines applying Japanese rules to HK or just wanting to ensure they keep all their passengers corralled in one place to avoid losing them!