“my mother was on medicare and had a quack doctor that was billing medicare and then billing her for the difference”
Based solely on your statement, the quack doctor was obeying the law.
Not true. From Medicare.gov
What You Pay for Part B Services
Costs for Part B services depend on whether you have Original Medicare or are in a Medicare health plan. For some services, there are no costs, but you may have to pay for the doctor’s visit. If the Part B deductible applies, you must pay all costs until you meet the yearly Part B deductible before Medicare begins to pay its share. Then, after your deductible is met, you typically pay 20% of the Medicare-approved amount of the service. You can save money if you choose doctors or providers who accept assignment.
He was billing her for the difference between what he charged for an offic visit and the amount medicare pd.