Actually @Mike-And-Jane0 Airbnb have an email people in this situation can use.
He can prove it’s his place and the tenancy doesnt allow for sub letting.
@Daisy181 tell your father to do a search on Airbnb Help/post on their social media for the email/contact them on their social media.
Why would he let these trespassers stay in the place. Why doesn’t he evict for sub letting abd change the locks/sue them in court for the money owed and possibly profits made.