One issue I see is that if the patient has a patient responsibility after the insurance is billed, but the patient isn't being billed, that is a violation of the contract. But they can't bill the patient's if the patient is told it is pro bono.
However, if the services are being provided it is not fraud to submit them to insurance, as long as all of the information on the claim is accurate. I can certainly understand them wanting to be paid if they can legally be paid. It allows them to provide even more services. If the provider is aware that even though she is providing the services pro bono that they are billing insurance and getting paid, and if it doesn't interfere with her private practice, then that's fine. As long as they are not violating any terms of the contracts they sign with the insurance carriers.