I don't see how that would be necessary under the law of most states. The only question is whether he knew that it would affect the contract between them, and he had no privilege to interfere.
My guess is that it was his intent to interfere. The best defense would probably be that he had a privilege, but if Allstate is held to have violated this guy's rights, I don't see how any privilege would apply.
There are no laws against "interfering" as you call it. Quite the contrary, you're free to voice your objections to either party in any contract at any time. If you have some factual information to divulge in the process, all the better. The only danger is if you invent something, at which point it becomes slander.