hi john,
we wonder how you set up administrator rights for this user. could you describe it briefly?
also, can this user see the sharepoint under admin in office 365 admin center?
generally, to access sharepoint admin center, we need to be at least a sharepoint admin. please check if this user is granted to be a sharepoint administrator:
in office 365 admin center, select users> active users. select this user, click edit user roles and then given him customized administrator role like sharepoint administrator.
if you just did this and the user still cannot access the sharepoint admin center, then please share the error message when he accesses the admin center with us, try some different browsers, and re-assigning this admin right to him.
regards,
april