Bad customer service

My mom bought a microwave Defy at the Silverlakes Makro on Sunday the 10 December she used the microwave and it keeps getting hot the wrong time over heating. She went back to Makro Silverlakes today 13 December 3 days later and was told that they will not replace her microwave and she needs to go home and not cook food in the microwave as it is only for heating food. Since when does a microwave being used just to re heat food. We will not buy from Makro again as its 3days return and they would not even listen to us. I am not impressed as now I will have to go buy another microwave just to cook veggies in. Look forward to speaking to a proper manager which will not be rude and brush us off.