If he didn't physically put the bag on the coach himself, it's very likely 'lost' in bagage handling somewhere. When he was coming from from Dublin a while back, my husband's flight to Leeds-Bradford was cancelled due to high winds - he was transferred to a flight to Manchester, but his bag wasn't!! Airline delivered it home a few days later.
Contact the airline - even if he DID load it himself, it's still possible someone took it by mistake (maybe both members of the couple picked up a bag, neither realising the other had collected it - it's very easy to do in the confusion of late-night travel disruption).