We have had similar. A very upsetting time. Didn't really feel that Airbnb support did much for us.
We too have strict cancellation policy. Our guest notified us a month before she was due to stay, that her daughter would rather go to Spain with her Aunt & Uncle than holiday with her so she she would be cancelling the booking with us so she could pay for her daughter's flight.
This woman hounded us for weeks and kept taking it to resolution centre. Each time support assured us that would be the last we hear.
The outcome is we will give her half of her deposit back if we manage to get another booking.
I have found this to be very stressful. Made me wonder what the policy is for ?