@Nicole1470To avoid any issue related to the calculation of refund amount, I would suggest a cancellation initiated by the guest on the date when the guest to leave. You can ask the guest to contact Airbnb support about it.
I had a long-term guest who booked for 3 months and stayed for 25 days. Her company re-assigned her to SF which is 35 miles away to work. I told her that an alteration would not be applicable in this case and asked her to consult Airbnb support, which she did. She finally canceled the trip and I got paid based on long-term cancellation policy. I did tell her if the dates freed were re-booked, she would be refunded partially according to the number of days booked.