Do NOT cancel. You will be charged (and penalized) far more than the guest. AirBnB will block the dates (meaning you can't fill them again) AND they will CHARGE you a hefty penalty fee AND they will downgrade your listing in search results. No matter how much sympathy you might feel for the guest, do NOT cancel. Tell them to cancel. They will be charged AirBnB's "guest service fee" and (depending on your cancellation policy) some/all of your booking fees. You can choose to refund all or part of the latter. In any case, do NOT cancel on behalf of the guest!!!