Can somebody plead insanity to get out of a contract?

I'm really sorry to hear all that, what you will have to do file a suit against the company in court. Now the tough part is you are going to have to prove that he was "insane" and thus unable to enter into a binding contract at the time he bought the silver. Proving someone is insane is not an easy thing to do you are going to have more than he believes some far out conspiracy theories. You should go and consult a lawyer and ask about particulars in your state. Hope this helps and hope everything turns out.