Not sure what you mean when you say "...the machine pulls the string.."
If he is finishing the knot by pulling on the knot with the machine tensioner, that is likely where the problem is--way too tight. The knot should be finished with hand tension, not machine, in my opinion.
The only good thing I can think of is at least this is not natural gut he is destroying. Gut, from what I have heard, is even more sensitive to tension when its knots are tied.
If I were you, I would buy a stringing machine and learn to do the work yourself.
Hope this helps,