Hello
Let the guest know you are sorry this happened and if they can get their car on the road, you would welcome them tomorrow. I would message them through Airbnb, so there is a record you are happy to honour the booking.
Say that if they want to cancel the booking, they can do so, and that Airbnb will handle in line with your cancellation policy. However, if they do this and you can rebook any days you will offer a partial refund.
If Airbnb decide to cancel under the extenuating circumstances they will need to provide proof via AA or similar of the breakdown.