Good answers. I would just add that it doesn't matter whether or not the client (your client's client) has reviewed the copy. The work was done and payment is expected.
While you wouldn't be able to hire a lawyer, I wonder if small claims court is an option (if kinder methods do not work first).
This is also a lesson to your client - one I learned the hard way as well. Ask for full payment up front or at least 50% payment upfront. Do not release the work until you receive the remaining 50% (okay, I never do this - but I've gotten better and now ask for full payment but end up with 50% upfront and then get the other half after I offer the finished product).
Hey, it takes time to get tough.
Janet