Hi @Sally11,
If it was a mistake, why don't you contact Airbnb and see if they can correct the mistake so you don't get penalized?
In any event, your guest needs to request a refund through their Airbnb account as the money is sitting there. They can either use it to book another listing or request a refund.
I would send them the following link which explains everything: https://www.airbnb.ca/help/article/170/what-to-do-if-your-host-cancels
Happy hosting!
Dave
David
Superhost Ambassador ~ Host Club Community Leader ~ Community Expert ~ Experienced Co-Host