The gas tank must be drained competely and can be easily done with a unit called a "gas buggy", most dealerships (if not all?) have one. The gas buggy just pumps the old gas out into its cantainer w/o needing to remove the tank. There is no drain on the tank.
DO NOT drive the car anymore as that'll fill the fuel filter with it and the car will soon stop running anyway, won't do any real damage but it'll make it more of a pain for the guy trying to restart the engine once gas is put back in the tank.