All does not stack up here! As has been pointed out in a few responses ...
First off, as twix123 said, a one digit error in the account number should result in an invalid account number. It seems implausible that the money has ended up in some unknown person's account, who has now spent it.
It also seems implausible, as PurplePolkadot said, that the landlord would wait so long before pointing out that payment had not been received.
That, combined with the flatmate's extreme reticence to show any evidence of correspondence or loss, indicates that the flatmate is trying to pull a fast one.
And, if so, evil_sheep is correct. Your daughter has paid her share. If the flatmate has "lost" it, that's their problem - not your daughter's. And if they're prepared to pay for it, I'd accept that ...