The guest has two options. @GKSuite0
a) they carry on with the booking but just stay for the first night but pay for two
b) they cancel the booking if they no longer want to travel. Obviously the guest should should cancel and any cancellation will be processed under the cancellation policy they booked under. It's up to you as the host whether you want to voluntarily refund some or all of the fee. They will still be liable for the Airbnb fee.
It sounds like you haven't familiarised yourself with how you use the Airbnb platform to market your STR business. For example you need to understand what happens if you as a host cancel bookings. You will face fines etc.
Do have a look at the Airbnb Help website it has FAQs on all aspects of using Airbnb for your listing