Bad customer service

My mom bought a phone for my son on December the 15/2017. He used the phone for few weeks then it went blank. She took the phone back to the store on the 1st of January where she was told that it will take them 6weeks to fix the problem. She was advised that she will be contacted once the phone is back for repairs which never happened. She kept on going there to find out of its been fixed or not for so many occasions with no luck..only yesterday the 08 that she went back to check if the phone is back. she was told that the problem has been fixed,only to find out that the phone was NOT Fixed... and now they telling her again that it will have to take them another 6 weeks to fix the phone, which is totally unacceptable for a pensioner who was trying to pleese his grandson to be treated like this.. kindly assist me by resolving this query urgently