@Thomas196 You and the guest have two options:
1. The guest cancels their booking in the system, subject to your Cancellation Policy. And if you wish to extend a refund to them (and accept a loss due to their failure to get travel insurance), you can do so at airbnb.com/resolutions .
2. The guest contacts Airbnb to request an Extenuating Circumstances exemption from the cancellation policy. This would take awhile to process, and if the storm hasn't begun shutting down roads or airports the exemption might be denied.