I don't know what state you're in, but in California, that's known as "self procured care". If you can get a written denial from the W/C insurance company you can send it with a letter to the patient's health insurance indicating that the W/C insurance denied your services.
If anything, the patient may have to pay for his "self procured care".
Patients have a right to go anywhere they want, but the W/C ins. co. doesn't have to pay for anything that wasn't authorized.
The patient's Primary treating physician could request treatment at your office and get it retroactively authorized, then you could re-bill the W/C insurance.