The situation may be legitimate and of concern to them but on the other hand they may just be finding reason to terminate the agreement and get as much of their money back. This is especially true if they are putting all their complaints on the Airbnb message system making it appear like a disaster apartment. If Airbnb can see that you have been helpful, of if Airbnb can see that issues are being blown out of proportion, i would hope that Airbnb would honour the long term rental terms as per below. I agree with @Gerry-And-Rashid0, no matter what happens, do not cancel the booking yourself. Let the guest do it.
If the guest books a long term stay and decides to cancel the long term agreement during their stay, the guest must use the online alteration tool in order to agree to a new checkout date. Regardless of the checkout date chosen, the guest is required to pay the host for the 30 days following the cancellation date, or up to the end date of the guest’s original reservation if the remaining days of the original reservation is less than 30 days.