Is it permissible for a doctor to write in a patient's treatment plan that he performed a filling for him, when he actually performed crowns, so that the patient can benefit from the health insurance amount?

As long as the insurance does not cover prosthetics, the subscriber is not allowed to commit fraud to obtain something not stipulated in the contract. Therefore, it is not permissible for your husband to assist anyone who intends such deception, due to Allah's saying: {O you who have believed, fulfill [all] contracts.} (Quran 5:1). And due to what Abu Hurayrah narrated from the Prophet, peace and blessings be upon him, who said: "Muslims are bound by their conditions."