Wow I'm fairly surprised at the advice you are being offered . The guest who has a booking with you for this weekend confirmed in good faith .
why would they want to stay in a hotel when they have booked in an Airbnb and why would they want to stay in someone else's apartment when they wouldn't be covered by Airbnb
it's pretty outrageous that your current guest is asking you to cancel on a future guest so they can extend their stay.
my take and what I think is the morally right thing to do is to advise the current guest that you can't extend their stay as you have a confirmed booking with another guest @Rosalie77