Most likely you can get a replacement thread insert from the kit seller at no cost. You, however have the cost of shipping the pen plus the cost in time devoted to eval and repair. It appears likely the user overpowered this fine writing instrument. What did he say? If he claims "normal use, not his fault", you have to decide if a freebie repair would be good policy, and result in benefit to yourself (reputation, repeat sales, etc.). It is reasonable to charge for repairs, but for myself, I have always done repairs at no cost to the owner. (I basically offer that verbally during my sales pitch anyhow.) I have had repeat sales with some and not with others, after performing free repairs.