Once you have configured the custom relationship permissions, you will have the ability to give this custom relationship visibility into private Updates 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 Profile of the user they now have visibility into. The Profile can be found under the "My team" section within the people navigation.
Here, they will be able to see any Updates (private and public) this employee has written.
As a reminder, this gives only visibility. The user will not be able to take action (such as delete, comment, or review) on an Update.