My opinion is that she set it up, it didn't work at first, she didn't use it, forgot that it existed, and here we are.
reply> Apple devices share fingerprint matching details and another device had her details
I looked into it quite seriously for windows thinkpads, unless Apple do it differently, you cannot share fingerprint, they're in a local chip and never move.
So how does TouchID on an external keyboard work without having to re-set up fingerprints?
replyPresumably the fingerprint data is stored in the Mac's Secure Enclave, and the external keyboard is just a reader
replyThe reporter lying or forgetting seems to be the clear answer, there's really no reason to believe it's not one of those. And the distinction between the two isn't really important from a technical perspective.
replyFingerprint security being poor is also unlikely, because that would only apply if a different finger had been registered.