Concerning euthanasia of a cat, what do you consider the best method?

I was speaking with a friend of mine the other day, and he told me he had his sick elderly cat euthanized. I thought he took his pet to a veterinarian, but he said he had a friend come to the house with CO2. I was shocked, because I live in New York State, and here it is illegal to use any gas to euthanize a mammal. The animal has to be sedated, and then a lethal dose of sodium pentobarbital is administered. I told him that, and he just scoffed at me. I told him that rather than CO2, which is just another form of asphyxiation, shooting the animal would have been more humane. So, what are your thoughts? Is CO2 more humane than shooting the animal? (I believe the only reason he had his friend come over was to avoid paying vet bills).