Are these guests already in your property? If no, then since she will no longer be party to the booking, she will need to cancel the reservation which will mean penalties for her according to you cancellation policy unless the reason she can't stay is due to extenuating circumstances. If this is the case, she will need to contact Airbnb to explain the situation, If they accept the reason as extenuating, they will confirm what proof is required and if they will waive any penalties.
The friend will then need to book the reservation in her name to be able to use her card. This is also vital otherwise if the guest who is currently booked is not one of the guest who will be staying in the property then it will effectively be 3rd party booking which Airbnb do not permit for personal travel.
If they are already in your property, these might a bit more tricky and to be honest I won’t personally be changing payments after the guest have already checked in. If there are any problems with the new payment method they might be out the property before it is resolved and you risk not getting paid. If the main guest wishes to leave, I would suggest she arranges to collect payment directly from her friends, after all that would have been what she did when she originally booked so she will just be asking for a little extra from them.
Also if they are already in the property and the main guest is leaving, I would probably contact Airbnb or guidance, If there are any issues down the line, how would this change impact you/ any protection you would get from Airbnb.
