Mom's being abused in NH-Rehab by a CNA. How would you handle this?

My mother is 96 years old and has dementia. She has been in a NH-Rehab because she stopped walking. I've had her personal HHA go there every day to keep her company and to have a familiar face around. Her personal HHA has been bathing, dressing, toileting her, and making her bed. Whenever she asks for help, because she can't handle my mother alone, she is told by the CNA that she's busy and will help her later. Then she never comes. On the rare occasion that the CNA does come to help she is abusive and disrespectful to my mother. She handles her roughly and barks orders at my mother. When she puts my mother on the toilet she drops her instead of lowering her gently. When my mother's HHA got there this morning she found my mother in bed with only a diaper on. My mother's personal HHA took a picture of my mother so I could see what was going on. I was furious and left work to speak with the Director of Nurses. They took down all the info and told me that they take this very seriously and will investigate the matter. They said that this aide will no longer take care of my mother. Any helpful suggestions will be appreciated.