The thing to remember is that states can provide more protections than the Federal Government but not less. So, under the guise of public safety, the state can further limit your client's access to monkeys. If there were a constitutional right to own a monkey, then your client would have a shot. Because of the 10th Amendment, I don't even see a viable due process violation. Of course, I haven't had Con Law yet, so take it for what its worth.