Low quality manufacturing after service

My fiance bought an engagement ring in December 2021, which i had to take back to re size and as such have only worn for approx 1,5 months. Had to return the ring on 22 March as one of my diamond had fallen out. Today the advised me that it would cost R865 to have it fixed!! I feel this is rediculous as its virtually brand new. I feel it is unfair to expect me to pay and feel the defective product should be taken back and credited or exchanged?