Hello @Leigh87,
As guests don't pay the host, they pay Airbnb, point out to the guest that all payment issues are a matter for them to discuss with Airbnb. You could mention that Airbnb has their money and doesn't pay the host until after checkin. There's a contact link for Airbnb support in the reservation confirmation email that guests receive.
If the guest availed of the pay less upfront option, whereby they pay Airbnb 50% at the time of making the reservation and 50% later, the payment page will have displayed the payment schedule when they were about to make the reservation.
For your own information, the remaining balance will be charged to their original payment method a few days before checkin.
Steve.