@Damian261 Everyone seems to be assuming you are talking about a regular (but against Airbnb TOS) third party booking, but I get the impression that you are aware that the person who showed up paid the booking guest to stay and that booker is making a profit off of subletting? If so, you need to report this to Airbnb- that the account holder has booked fraudulently and is turning around and subletting to others.
And you do not have to let the person who is there stay. They may be an innocent victim, or maybe not, but tell Airbnb this booking needs to be cancelled by them with no penalties to you.