As a host, you are not responsible for taking care of money matters for your guest. The only way to rectify this is to instruct your guest to contact Airbnb directly, who will be able to explain the charge to them.
Also, to note: the entire cost of the reservation is charged when the guest books, not just a deposit. Could this be why the guest thinks they were charged more? Or maybe it's a currency conversion problem? Lots of possibilities and best for them to speak directly with Airbnb to hash it out.