Let's see. The original client is using your copyrighted work, and you never got paid?
Check with a lawyer. If you didn't get paid, he may well not have any right to use your copyrighted work. Therefore he has no ability to sell the right to your work to the original client. If so, then the original client is now in violation of your copyright.
If so then the fact that you are in a position to cause trouble for the original client might accomplish something useful. But think carefully about how to use it in negotiation before going there. (On the plus side you may poison that original client against that person. On the negative side, you're unlikely to recover much after legal expenses.)
Check with a lawyer. If you didn't get paid, he may well not have any right to use your copyrighted work. Therefore he has no ability to sell the right to your work to the original client. If so, then the original client is now in violation of your copyright.
If so then the fact that you are in a position to cause trouble for the original client might accomplish something useful. But think carefully about how to use it in negotiation before going there. (On the plus side you may poison that original client against that person. On the negative side, you're unlikely to recover much after legal expenses.)