She called saying "they're saying 'the lug nut broke'"; I was thinking (and asking) "how is that possible (it can't possibly be a defective lug nut)?" 10 minutes later "the stud is broken"; 5 minutes later "they [Costco] admitted to breaking it".
The resolution was to have a local shop replace the lug stud and get reimbursed for the repair cost: $125 (the part might have cost $2)!
I chalk this episode up to them hiring warm bodies lacking any mechanical experience or aptitude (to the extent of not being aware of "righty tighty, lefty-loosey") and handing them an impact wrench.