A power resistor is not going to change resistance significantly when its hot or cold. Not sure what happened when your friend moved it off the water hose but it wasn't because of the temperature.
The reason you want to use a coil with an external resistor is so that at start, you can bypass the ballast resistor to get a higher voltage to the coil. The problem with the internal ballast coil is that you will get a much lower voltage while the engine is cranking.
