your guest has to cancel the reservation and ABB will refund him according to your cancelation policy. You don't have to do anyting.
If you get a message like this: "your guest xx wants to cancel, do you agree?" DO NOT AGREE! If you do he will cancel on your behalf and you will be punished the same as you cancelled
Thank you very mch for the warning as it had occured to me how this would impact on my rating as a host. He has not been in touch (more than a week) - how long should i wait before I can keep the payment?