If he was helping as a friend then they are just fishing for some money. Your homeowners/renters insurance will provide legal and coverage. If your dad doesn’t have insurance he’d probably win in court but an attorney will be expensive
Why would homeowners or renters insurance cover unlicensed contractor work done on someone elses home?
If he’s helping the homeowner as a friend, I don’t believe that would constitute contractor work. It’s homeowner DIY.