I wonder how your guest got on in the end, Natalie? I've had the same thing happen to me when trying to pay the balance due on a booking where the reservation has been made on a different card to the one I wish to make the balance payment on. I have spent ages this morning talking to both Airbnb and my credit card company. They lay the blame with each other.
However, I think the credit card company is right as they are giving me the security code (one -time passcode) to enter on Airbnb's website and it is Airbnb's website that is not allowing me to complete entering all the digits. I've done this so many times and it just doesn't work. At no point in my discussion did Airbnb tell me that I'm not allowed to change cards for a single booking but I get the feeling if I hadn't tried to change the card it would have allowed me to pay.
There is also an error message that appears as follows:
"Unfortunately, a server error prevented your request from being completed. Airbnb may be undergoing maintenance or your connection may have timed out. Please refresh the page or try again."
Perhaps this is simply a bug in their system. Not sure what's going to happen come payment day...
Any answers anyone, please?