If a panel is full it is full. To install a tandem is not only against code but a denial of the problem at hand. If you had seen a receptical with a broken top would you have fixed it or just told them to use the bottom? If the panel is so full that you could not fit another in it, it's time to upgrade or expand the service. If the client wants a reason why it is for their own safety, the safety of the property and your peace of mind that you did not add to the problem most of us are faced with daily. I would not want to imagine the world without the nec and even if I do not personally agree with each one of them, I do have to take my hat of to them for what they stand for.
