I think it's the fact that the bills were probably underestimated so although your Mum was paying regularly it would not have been enough to cover the actual amount so an outstanding amount has accrued. Shame but I think you're fighting a losing battle. Sorry to say that !My mum’s balance was in credit. She was very careful about things like that & she would save up any correspondence for me to look at but all the bills would have been paid via direct debit until her death so she was up to date paying the bills but they were always estimated usage based on her previous bills when the property was occupied.
I would double check the final readings were correct.