It would be easy to help you if you can say exactly what data to which table you want to display as you have already shared the TO graph.
I've been working on it, so I've included an updated graph. What I'm looking to do is have a Drop Down list in my Invoice table that I can select a last name from the Client table. Once selected, the full name (First & Last) and address populate on the Invoice table.
From the relationships shown, your invoices table will need to show "invoice copy" in Layout Setup | Show Records From. But if you have an established layout based on Invoices, then this could make the relationships shown workable for your layout but keep existing parts of your layout from working.
See this thread for how to get your data from a related table to appear. (No portal needed for this relationship since you would have only one client for each invoice.)