Spurious user on log-in page

A colleague who is nearly computer illiterate, tried to create another local user on his home PC (I do not understand why he did this as he did not want to have another user, only himself on a local account). He is the administrator for the PC but will not use a Microsoft account. Having clearly failed in his mission, he now faces the situation where a spurious user named "customer" appears on his log-in page. In order to get to his account he has to enter his user name and password. But every time the PC restarts it defaults to the "Customer" account.
I have searched in vain for the Customer user name but it appears nowhere.
Can't see it in the registry, in the manage user accounts or on the start menu top left.
Can anyone advise me how to find and delete this spurious account?

Thanks for the reply. Sadly, as I cannot see the user name "Customer" in the user profile, ergo I cant delete it. The only account that appears is his administrator account (in his name).
If I delete this master account that would make life a little difficult?
Should I create a new administrator account and delete his account - will that help?

Thank you to "essenbe" and "Brink" for your helpful replies. I located the user on the C: drive and yes he did use netplwiz so that is fixed also.