ADT keeps billing my mom's closed account

My mom decided to close her account the beginning of June 2015 after experiencing billing problems. Her last payment in full was in May 2015 and she requested that the account be closed. In June 2015 money was deducted by the company and when she called to enquire about it, she was told that her account did reflect as being closed and she should reverse the payment. Unfortunately now she has received a bill from ADT asking for payment from June 2015 till now. How is that possible since her account is supposed to be closed?