I don't understand what has happened because (AFAIK) you can't use contactless transit mode to travel on the Airport Express. From what I have read, this will only be available from 2026.
Also, there are no ticket barriers at the Airport station, so even if contactless transit mode was available, you wouldn't have to tap your card at the airport. That's how it works with Octopus.
(Note that the Airport Express only goes to Hong Kong station, you cannot access Central station.)
USD 26.41 looks like HKD 205 which is the price of a round trip to Hong Kong station.
USD 34.79 looks like HKD 270.
She could check her transaction history here to see what it thinks she has done
https://contactless.mtr.com.hk/side-check
However if she's just purchased some sort of tickets at the counter twice (??) then the portal wouldn't show anything...
I have used contactless transit mode on the MTR recently and haven't had any issues, including receiving the promotional rebates.