Perhaps the OP's nephew was flying from HKG earlier this week?
There were two days earlier this week which during the late afternoon, all flights departing HKG were "cancelled" due to protests at the airport.
A lot of these "cancelled" flights (mainly international flights operated by foreign carriers) ended up departing as normal with a less than full load (mainly with transit passengers who originated in other cities and only connecting airside in HKG).
When these flights were "cancelled" (which is not really the case), departing passengers originating from HKG were prevented by the HK Airport Authority from clearing security into the secure area as there was a lot of chaos caused by the protesters.
It might be possible that when the OP's nephew reached the check-in counter at HKG, the agents saw that there was little chance he could make it pass security to catch his flight so re-booked him onto the next flight 24 hours alter. Due to the poor English skills of the agents in HKG, they might have incorrectly said something along the line of "overbooking".
As this delay was caused by civil unrest as opposed to a mechanical issue or an actual overbooking issue, the OP's nephew does not need to be provided with meals or accommodations.