Pathetic shocking service

My mother had purchased a faulty microwave, since the microwave had been previously returned to the store before the box was damaged and she had to throw it away when she got home. The microwave worked for 2 days while giving out cold heat. Until it stopped completely. My mother is being told by the store manager she can not return it because she no longer has the box, the slip and physical microwave is not proof enough????? I'm going to be taking this further. Who's fault is it that the box was damaged and couldn't be kept.