PLEASE ADVISE HOW TO MAKE PAYMENT AND GET TICKET COLLECTION AT HKG INTL AIRPORT

My husband will be arriving off a CX flight from Australia at 1810hrs on Monday. He needs to then get the
Airport Express MTR to Kowloon and Taxi to his hotel. Could you please advise me how and where he can
purchase a rail ticket when he arrives at the airport as we won't be able to pre-order/online book in advance.
He will be needing a return MTR ticket to take him from Kowloon back to the airport the following day for his flight back home. Can he purchase and pay for the return sector Airport express journey at the same time?
Thank You for your help