There are a lot of assumptions here. The specific frequency of the microwave oven is tuned to be absorbed by water molecules.
Yes other materials, particularly metals, (shape and size matter too) can absorb the RF energy as well, but a lot do not, and the RF passes harmlessly through them.
Just like the massive amounts of RF that is going through you right now, every second of everyday. It is everywhere, but the wavelength is something that ignores almost everything you are made of.
He's a total fucking moron if he thinks military service dogs aren't well taken care of.