@Vicki451 Really sorry to hear that happened to you. Sounds like a very stressful situation.
Normally under these circumstances, the guest would not be refunded for the first month under the terms of the long-term stay. See the cancellation policy at Airbnb Cancellation Policies. However, as you say, it seems that the guest will be angling for a full refund by claiming the place is "dirty". Whether or not you will be protected depends very much on the customer service representative that gets involved in the case. If they decide to side with the guest, there is definitely a possibility they could be fully refunded. However, they are supposed to contact Airbnb within 24 hours of discovering an issue, so if they haven't done that, it could work against any claim they may try and make.
First of all, bring all communication back onto the Airbnb platform. Be clear that it's the only way you will communicate with them. Offer to come over to clean immediately: make sure you do this via Airbnb messenger, if you haven't already. If they are not open to you coming in to clean the space to make things right, encourage them to cancel the reservation if they are not happy: that will at least open your schedule to other bookings. They will be refunded for any days after the first month if they cancel, but if they're going, the more time you have to get new guests, the better. One carrot to dangle is letting them know that if you get a rebooking, you will refund them for those nights. It might get them moving, if nothing else.
Good luck with your guest! I hope they're not able to get an unfair refund. Hopefully you will get a replacement booking.