@Clay123 If the guest would like to end their booking early and relocate, you can send them a Change Request with a revised checkout date, refunding them for the remainder of the booked period.
If they prefer to stay (or if the weather is still preventing them from leaving) you can offer them a partial refund, perhaps 50% off the nightly rate. But I hope the house is at least equipped with sufficient disaster supplies so that the outage doesn't cause further safety issues.