I would let him move on and find another stringer. If he's the only one of your customers with this complaint then it's clearly not something you're doing wrong.
You could recommend another stringer you know and then check with him after he strings for this guy and see what happened.
Head YouTek Prestige MP (18x20) - Wilson Nat Gut 17 (1.25) / MSV Co-Focus (1.18)
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