If your boss tells you to vandalize a car or steal some candy, you are liable for choosing to break the law. Your boss telling you to do something doesn't get you off the hook.
Now chances are, if the copyright finds out and sues anyone, it'll be the corporation instead of you. Why sue a lowly employee instead of a corporation with the bucks? The corporation is responsible for what you do. If you decide to do what your boss is asking, make sure you keep a paper trail of evidence that this is something your boss is telling you to do.
These days good jobs are fairly scarce, and I understand needing a job bad enough to consider certain law violations, and, honestly, unless the current client will use the materials in a high-profile way, I'd probably do what the boss is asking (this isn't legal advice or advocating anything), but if it's high-profile, then no freaking way. If the job market was better and jobs like this not being shipped overseas left and right, then no way, and I'd start looking for a new job and play up my integrity in interviews.
Basically a paragraph to tell you that you're not a bad person for considering doing what you know is wrong, but shame on your boss for demanding this of you. He's wrong.
Is there any way to talk with the client yourself about this? You know, casually dropping how there's the possibility they won't be able to use the material since it contains other material that has copyrights on it?