Once you have configured the custom relationship permissions, you will have the ability to give this custom relationship visibility into the private Goals of the employee they have been set to oversee.
To view this, the person who has been given this visibility and applied this custom relationship, can go to the "You" page of the user they now have visibility into.
Here, they will be able to view any Goals (private and public) associated with this employee.
As a reminder, this gives only visibility. The user will not be able to take action (such as delete, edit, or update) a goal.