Hello @Niang0
So the guest forgot to cancel his reservation and is now asking for a full refund but am I right in thinking that they did inform you of their intention of cancelling a few days before the arrival date?
Because they didn't cancel, your calendar remained blocked giving you no chance of getting another booking. But, when the guest told you that they wanted to cancel, were they still inside your cancellation window to receive a full refund?
If so, I would have messaged the guest to tell them to cancel their end so that they could get a full or even partial refund. They may have mistakenly thought that by informing the host, that this was the normal procedure??
Airbnb will ask you if you want to refund the guest or uphold your cancellation policy. It will be your choice.