@Boo1, I looked at your listing and you have a "moderate" cancellation policy in place.
I understood you to say that now the guest has agreed to cancel - so there actually are no service fees involved for the guest! ( Airbnb has apparently just changed their rules on that, which now makes things a whole lot easier and hassle free .) So your guest should simply cancel, no fees - and everybody will be happy! Just go check on your own page , have your guest also look at it as well , as you'll see this now clearly under your cancellation policy:
they just need to make sure to cancel your place first - and only after that select another lodging:
Just tell your guest this - Good luck, hope that solves your problem!