@Martin-And-Lesley1 Firstly do NOT cancel on their behalf. You will be fined, a negative review will be left and the dates will be blocked in your calendar.
Tell the guest they must talk to Airbnb. Customer service will probably believe them if they say they have covid and cancel the reservation so you will lose out but at least there is a chance someone else will book.
Guests have your phone number as soon as a reservation is confirmed.
I would tell the guest to cancel. Your normal cancellation policy will apply, unless you decide to offer an additional refund OR the guest persuades Airbnb to refund them due to "extenuating circumstances."