Is this theft?

A friend of mine recently paid her Virgin Media bill over the telephone to a Virgin Media employee using her debit card. The bill was �86. The money has been taken from her account but there is absolutely no record at Virgin Media that she ever made a payment. As you can imagine she was furious and faxed a copy of her bank statement to prove that she made the payment. This was 8 days ago. She has contacted them several times and they keep saying they can't find the missing payment. Is this theft in the eyes of the law? she has even recieved a final demand. What steps can she take because Virgin Media have no right to be stalling on such a serious error. Warning to all Virgin Media customers, be very wary of paying your bill over the telephone.