Powerflow pinetown

Terrible workmanship and terrible service. I took a customer's vehicle in to get the downpipe modified to fit correctly due to changing turbos before the vehicle was due to go to PE. It was only 3 welds that needed to be done and tighten 3 nuts for exhaust flange after putting sealer on. The car sounded to be blowing very slightly soon after leaving and didn't think much of it. I then had it brought to my attention by my customer after delivering the car to discover that 2 of the nuts where no where to be found and was leaking heavily by the flange and by the welds they had done. I phoned powerflow today to make them aware of what had been left out and the issues I had and to my shock he said they never leave nuts off and they must have vibrated off while driving, me being in the motor industry this is utter nonsense a copper exhaust nut has a self lock and can't work its way loose once it's been tighten and why would only one nut still be on and tight and not all have come off. I wasn't told an amount of how much it would cost todo and was ignored after asking 3 times for a price and the response was once its done we can talk and was then told R800 at the end which I paid even though was quoted less at other shops but now has cost me an additional R650 to have sorted down in PE and no responsibility has been taken by the owner of powerflow pinetown, you mean to tell me every 1000kms my exhaust nuts will come loose?