Like others have said, tell the guy to stop pulling the knots tight with the stringer. Either that or tell him that if he insists on doing it to do it right... i.e. tap to start the tension and then tap to stop it a second later. You NEVER let the tensioning head pull all the way like it does when you're tensioning a main/cross.
Better yet, get a new stringer or just do your own racquets. If it's that expensive to buy string there, imagine how much the guy is probably costing you in labor. Buy a machine and do it yourself... you can get a good 6-point drop weight for about $400 from a reputable company and upgrade the tensioner later to an electric if needed.