His general business liability insurance probably doesn't cover hitch installation/reinstallation. That's not a legal/law issue. That's an insurance liability coverage issue.
My general business liability policy has 50 pages of exclusions in it. They've written it such that they can deny coverage for practically anything. So I just make sure that everything I do, is top notch quality and overbuilt design. Chances are, if anything goes wrong, my general business liability policy would leave me hanging out to dry to fend for myself. So I make sure nothing happens...