Actually, if someone built me a coil in a vape shop, and it shorted, and I was injured, there's very little question that within a few months I'd own the vape shop. LOL.
HE should have a "waiver" drawn up - wouldn't have to be too complex, just a pad of paper the customers have to sign that says something to the effect that "I understand that using low resistance coils in a vaping device requires knowledge of battery safety, and that there is inherent risk to any use of rechargeable batteries. I agree to waive, release and discharge _____ vape shop harmless from all claims, liability, loss or damage arising from the use of custom coils." Or some such - from a lawyer.