Broken Stud Bolts

I purchased a set of mud grips 2 months (4500 miles) ago and my wheel broke off, while hauling my 6 horse trailer... The tire shop that put the wheels on is refusing to take responsibility for it saying "there is no way overtightning the lugs would cause the bolts to brake". I don't believe him, but I am not a mechanic. What should I do? There is slight damage to the tire that I just finished paying off and I may need a new one. There is a $100+ towing bill and the cost of labor and parts to put the damaged tire back on. Am I right to want him to cover the costs? Has anyone had this issue before?