@Andrew919
With this quirk of timing, you will have a guest reservation amendment that you can't fulfil as the date is no longer available. I'm suprised the airbnb system doesn't inform the guest automatically that their amendment request is invalid, maybe another quirk.
You can only decline the amendment and inform the guest that the requested date is no longer available - the original reservation for the guest will then remain. It would be up to them to decide if they initiate a cancellation or proceed with the reservation.
If the guest wants to cancel, make sure they cancel from their end, don't you do it.
Perhaps the original booker might change plans once again though, at least you have a confirmed booking from the second guest.