Selling **** silver necklace

Did bought a sliver necklace and pendulum from them late in December. It was not an expensive neckalge but when I bought it I did ask the shop owner if the silver will come off or fade in anyway. She has informed me that the necklace is some specific silver and will not lose colour or come off. I bought the necklace and pendulum as a gift. 5 days after my friend wear the silver necklace the silver colour became almost black. The pendulum was ok. After I returned to SA she took the necklace in to clean off the blackness. Upon trying this the silver just came off and it looks like copper only. I went to to confront the store owner and was just told they don't know why it did it but will not compensate me in anyway and I can do what I want.