@Jamie20 Yes, the first booking charge was made at the time the reservation was confirmed.
As for the alteration to the reservation with the now expired card, the guest has 24 hours to update their payment method. The alteration won't go through until this happens.
You are not supposed to accept cash, but you are entitled to be paid for the extra guests. Hopefully the guest will update their payment method soon. If they don't get it resolved by the time they arrive, you can also use the Resolution Center to collect the extra guest fees, assuming that at some point they've updated their new credit card.
Alternatively, if the guest fails to pay for the extra guests, you can cancel the entire reservation (not sure you really want to do this) but I would contact Airbnb first to get their advice. You don't want to be penalized for a cancelled reservation!