The only problem I see is maybe the thread isn't the best quality. I would ask for the customer to return the quilt at your expense and then remove every loose thread, bury the tails and take out the double back up stitches at the miter. A more experienced machine quilter will see every flaw in another's work but the shop's evaluation was uncalled for. The quilt won't fall apart. I am afraid you lost money on this one to make it right.