Thank you for the post.
The [Full Access] account on a file in FileMaker Pro 12 will have access to File > Manage > Security > Accounts. This is by default and cannot be changed.
If a particular account cannot access File > Manage > Security, then that account is not [Full Access]. A FileMaker file in the ".fmp12" format must have at least one [Full Access] account. You cannot delete the [Full Access] account. If an account is the only [Full Access] account and a user attempts to delete the account, the program will throw an error and not allow the account to be deleted (see screenshot below).
The only caveat to the above statement is that someone with FileMaker Pro 12 Advanced could use the developer tools to strip the only [Full Access] account from the database file. This cannot be undone, and a previous version of the file would be required to once again access the account settings.
The suggestion would be to contact the person will the [Full Access] account because without a [Full Access] account, no changes to account settings or privileges can be made.