If he has an in-network contract with that insurance company, he's almost certainly in violation of it. I've read and negotiated a lot of managed care contracts, and here are some examples of typical contract clauses that he'd be violating:
1) His contract would have agreed to in-network rates. If he's charging the patients full billed charges, that's a contract violation.
2) His contract most likely has provisions about submitting claims to the insurer on behalf of the patients.
Additionally, the insurer will send payments to in-network providers. He is going to have a LOT of angry patients demanding refunds after the patient finds out that both they and the insurer paid the provider for the same service. (If I were one of those patients, I'd file a complaint to my insurer and every regulating board I could.)
If he wants to do business like a non-contracted provider, then he needs to BE a non-contracted provider. He can give notice and terminate his contract.
(There will be a notice period where he'll still have to bill the claims until his official termination date. His contract will spell out what he needs to do to terminate the agreement and how long his notice period will be.)