Hello @Martin McGinley (Martin) !
Welcome to Microsoft QnA!
Let the user go to
https://mysignins.microsoft.com/security-info
From there change the default and also can delete the Authenticator
https://learn.microsoft.com/en-us/azure/active-directory/authentication/concept-mfa-howitworks
I hope this helps!
Kindly mark the answer as Accepted and Upvote in case it helped!
Regards