Horrible Service

My aunt went to buy a stove thermostat at their shop on the 27t h of jan 2015, she got home tried to use but the product didn't work, when she went back to the shop the following day to let them know that the thermostat wasn't working, she was told she shouldn't have opened it and so they can't take it back and there's nothing they can do about it. and the person that was assisting her was very rude towards her. What we would like to know is how was she supposed to know whether that thing works or not if she doesn't open it and try to use it. Also how is it legal in south africa that there's shops that say you can not return a product you have bought from them even if you are not happy with it.