Call them back, Erica, and steel yourself for an argument. Stay calm (I know this is insanely frustrating) and be sure to keep your communication with your guest within the airBNB messaging system. Be prepared for airBNB to list a bunch of trumped-up reasons why the guest wants o leave early. THey may have invented a whole slew of nonsense.
Check your calendar to see whether or not airbnb has already freed up the canceled days when they okay's the guest's equest. If not, use this as one of your points when discussing how ridiculous it is that they're ignoring your cancellation policy.
If airBNB wants to refund the guest, that's fine, but it should absolutely not come out of your pocket.