Additional Administrators of sites are not allowed to see the account history of the "owner." They also cant change the owners password, nor touch the owners other websites.
Administrators can see the balance owing and can top up the owners account balance.
Administrators can edit content of sites they are permitted too.
If you want the admin to be the site owner, change ownership to their uid. And make the old owner the admin.
Invoices will be emailed to the site owner and 1 additional email address for the accounts dept, but that additional email address cant edit site.