Has anyone tried to change both account and computer names following
the methods set forth in the support articles about this topic?
Is this 'User' account an Admin account? There are procedures where one
can make a new user account and move content into it, then delete the old.
Some owners will set up a Standard account, and only log into Admin to
perform updates & install new software. That would be a security measure.
Perhaps a permissions or privileges issue has developed; maybe to see if
repair disk permissions or other ideas may help. Usually when a name was
changed, there is a confirmation process. Especially if the computer account
relates to an AppleID and online store, etc.
The inquiry and reference to Parental Controls was added to my reply due
to the practical issue where I could have told you (or her) how to work around
it and if the computer is owned and managed by an adult, that wouldn't have
been fair to their interests or edicts as parents.
Anyway, perhaps someone else can answer your question; after asking the
questions that need reply in order to determine what was done in the OS X.
Good luck! 😐