You may want to have your employee timesheets approved by an external client user. In order to do so, you will need to have a user account for the client user, which you can assign as the approver for your employee.

To create a client user, simply follow the same process to create internal employees. When creating the user account however, ensure that ALL administrative and standard roles are unticked. Doing so will ensure they do not have access to create timesheets and hence see any confidential information about your project and clients.