A contractor’s work should be able to stand the light of day. A homeowner may have an idea for project and hire a contractor to complete that project. I don’t find it unusual for a homeowner not to be knowledgeable about construction details to invite parents, aunts and uncles or knowledgeable friends to stop by and examine the work. The homeowner may have been strapped for cash and had to use expensive materials. None the less there would be an expectation of quality workmanship. Too bad your dad left, a discussion with the contractor may have been helpful. Now it seems adversarial.
Non compliance with code? Not sure of the codes referenced. Again, to me that seems like a contractor responsibility. I’m sure the homeowner’s insurance or the contractor’s insurance would have expectations of work being compliant. The homeowner may find out when they later try to sell the house, an inspection may find those items and they might need to be corrected to complete the sale. Although drywall can hide a myriad of faults.
if you think your dad can be cool headed, I might suggest a meeting at your home with the contractor. Try not putting contractor on defensive. Pose discussions as questions, why was this done this way. Not, why isn’t this done right.