If the custies quote you as saying it's illegal you'll look bad, but i think in these circs the customer could be liable if there was an accident because they are allowing it knowingly.Their household insurer would be the best ones to ask because they will be the ones having to pay.
Without meaning to wind any ladder users up it's a fact that the health and safety boom is led by our claims culture.To some, if you believe the injury lawyers adverts, falling off a ladder could be the best thing that ever happened to them, and the start of many miserable years of litigation for the householder.