your mom should be seeing her endocrinologist to manage her diabetes. and if you are certain he is abusing his prescribing privileges you might wish to pursue bringing this to light by contacting the licensing? board of your state and filing a complaint.
You are the patient's advocate and have every right to question her medications. A 2nd opinion from a different doctor will go a long way in proving your point.
Her Endocrinologist should NOT be *arrogant* with a concerned family member, he should be willing to conference with the family and explain your mother's medical records and his reasons for prescribing a controlled substance ongoing for her.
BTW, it is your mother's RIGHT to obtain copies of her medical records, they should not try to withhold information from her/you. But she will have to authorize you as her agent /guardian if you do not have medical POA.