Dilemma - ours is v.economical and uses only 16 litres of water to do it's whole wash. When you consider that it will hold an entire day's washing up where you might use 3/4 bowls of water to do it by hand, much better to use the dishwasher.
No running gallons down the sink while you wait for the hot to come through.
V.good if you are ill because it gets everything sterile.
If he barely does the washing up then he has no say - just order one and organise a plumber!
Seriously though, my DH is very eco-minded and he did his research and insisted we got one.