Diesels can also get a pretty wide range of fuel economy, too. So depending on what he's driving, and what you're driving, its possible you're getting 4 times the fuel mileage he is. But you'd have to be driving a hybrid, and he'd have to be driving like a Hummer....
You should be able to check fuel mileage on the net. But unless he's driving something that's been turned up or chipped out the wazoo, or is ancient, he should be able to pull at least 12mpg out of almost any diesel pickup. At the very least.