Terrible service

My mom is a pensioner and they keep telling her that she never paid yet she pays it every month. we went to the branch and gave proof of the payment, I then had to E mail through as well, first they said they need May month now its again August. we paid twice to *** and *** still they say she owes money.she is now in hospital and this is causing so much stress. Please investigate and see how you can resolve this