It is the responsibility of gate staff to ensure no drunk person boards an aeroplane. However unless the pax have been notably rowdy they almost always find it easier to turn a blind eye and make it the crew's problem. No doubt one of the cabin crew alerted the Capt who did the right thing.
This pax might well have slept through the flight. Equally he might not. But then he might have slept through an evacuation too...and killed people.
Sadly some airlines just allow them to board another flight when sober. IMHO they should always be made to buy a new ticket.
Expensive hotel? I hope only the honeymoon suite was available. Own goal, and hope the idiot doesn't do it again.