Thanks. I finally read all the way to the last footnote on Apple's page about WMA (https://support.apple.com/en-us/HT204087) and see that it says "Each time you migrate, your Mac creates a new user account.". Oops.
I ended up looking over the files on her Windows account, and it was a simple enough situation that I could just copy the Docs, Downloads, Pictures, and Movies contents to an external SSD, then drop those files into the corresponding folders on her new Mac account.
Next time, though, I'll know.