upgrade phone not working

my aunt upgraded with a sumsang A7 on the13/11/2015 at the above store and a week or 2 later the fone was not working.she took it back to th store and was told to bring it along with the box which she did lat week were she was told that the pfone looks fine and if she insist they take it back she might be made to pay.i called the store earlier spoke to a lady (white) didnt catch the name who promised me a call back on *** number which i have not yet received.now how does she continue to pay for a fone that is clearly not working (doesnt switch on or even respond when on a charger. help