MWEB is taking advantage of an old lady!!!

My 69 year mother took a package with MWEB this year April and had been paying on time. However, there are two months were she paid and mistakenly used a wrong reference. I have helped her to send her proof of payment almost every week but the bill keeps on coming back saying that she is still owing for two months. Now they have added some sort of interest to it. When i call them, they give me the same email address I have been using to send the POP. They are taking advantage of an old lady. You guys are disgusting!!!!