All posts are available for view at the front-end by default.

There is a way to restrict posts available at the front-end for view by the logged-in user:

Content view access restriction for selected roles

It does not include ‘by author ID’ feature though. I see that such option will be useful. I will add it to the some of the future versions.

Currently, it’s possible to use some tag or category as a workaround – setup such ‘Posts View’ restriction for role, assigned to this author.

If it’s not acceptable for your client I may provide a piece of code for functions.php in order users of some role will see at the front-end just the posts for which they are authors.