The doctors can explain it but their insurance prohibits it. You can get the surgical reports and the pathology reports and read up. Your primary care doctor may be able to help you with terms but doctors get very touchy when it comes to errors.
It may not be the surgeon’s issue, but the way the tumor was wrapped around the optic nerve which is not always evident on imaging, but the fact that they are not explaining how the tumor sat, it increases the likelihood that the surgeon made an error and/or the anesthesia doctor.
It is unlikely that your father will regain the sight in his eye. He should see a specialist eye doctor called a neuro opthamologist and have the doctor evaluate the optic nerve and the eye and he/she can explain more but the optic nerve is not one which recovers over time. That doctor will give him more information and tell him the odds of gaining sight.