Some people are just never happy. If I was in your shoes, I would ask for the string to be mailed back, and a full refund would be in the mail back to him, then put the strings in the give away section of this site, and never take another order from that individual again.
Everyone goes away happy, well except for you, your out time and material, but it is a business, and the customer is always right, even when they are completely wrong. Only problem is, that in their mind they are right, so it's best to go separate ways, and let the next guy deal with him.
Now this is just my opinion, but I do believe it will be echoed again and again.
You make a good looking string Josh, but I can honestly say I have yet to shoot one of yours. My next order will be from you though, and yes, I will listen to your advice.