We're expected to pay for someone else's mistake!

My mom has a contract with Cell C. She changed her bank account details at the beginning of May at the Chatsworth Centre store. Her old bank account was not closed. It takes 30 days for the account changes to go through. Its now the middle of July and we went into the Cell C store a few days ago only to learn that the first person to capture the new account details, did so incorrectly. My mom has constantly been receiving messages saying her account is in arrears and when we spoke to Sean at the Cell C Chatsworth store he told us that my mom will have to pay a penalty. He said he'll see what he can do, now my moms line is suspended? Why should we have to pay for someone else's error? This is horrible.