Billed for a Cancelled Contract

My girlfriend went into a Vodacom store to switch her contract to prepaid (her device and contract was fully paid for in May 2019). The store advised that her cancellation request could only be processed via the cancellation call center. Fair enough. She proceeded to call the center ( ********** ) on 1 June 2019, requesting the cancellation. The agent was quick to process a cancellation over the phone (SMS'es were received confirming this) however the agent did not provide her with a reference number for her call, nor did he explain the process at all - in fact the call was cut short and when she re-attempted to phone back ( ********** ) she received a pre-recorded call that Vodacom are not prepared to assist as she was a prepaid customer. From the moment the call cut all her data / voice minutes / SMS'es were clawed back and her phone was inoperative until she reached wifi to load prepaid. On Saturday 8 June she received an invoice for the Smart Top Up M+package, due on 30 June 2019, yet she had zero data / minutes / SMS'es for the month of June. When she inquired at a Vodacom store later that day she was advised to call yet another call centre ( ********** ) as the in-store agents cannot assist. The call centre would only be available on Monday. Earlier today (Monday 10 June) she called the call centre to inquire about the mis-issued invoice - the agent on the other end said that her query could not be escalated on account of the system giving an "error" - this, after being kept on hold for three quarters of the phone call! The agent also informed her that she would receive an SMS with a reference number to escalate the issue - 5 hours later and still no SMS. Absolutely shocking customer service! What does a loyal customer, whose account has never been in arrears, have to do in order to prevent this debit order from going off!