If the reservation is for 28 nights then Airbnb automatically applies the long term cancellation policy to that reservation irrespective of what your normal cancellation policy is.
Under the terms of the long term policy he's not due any refund if he cancels during a 28 night reservation.
You can, of course, decide to give a refund if you wish.
the articles below have some more information on changes to long term reservations.
Prepare yourself. Steve is right about the technical policy, but Airbnb is going to call you anyway to try and guilt trip you into an "alteration" to end the stay early for "fairness and goodness" and all that if the guest requests it.