Restrict access to multi clients

I'm trying to get an online system working that allows: Users to log in to an Owner organisation ie firm of accountants. Owner to display list of clients to work on.

So I have that working with User: Id owner_id

Owner Id

Clients Id owner_id

User has owner relationship and owner has client relationship.

Thus User can choose from a list of clients to work on.

HOWEVER what I want is to be able to restrict the clients that the User can see.

How do I go about that?