Other than a good tune, it's generally hard to recoup money spent on gaining mpg a lot of time it's just cheaper to put it into the tank. Gains are generally small especially for dollar spent so you would have to be putting a lot of miles on to break even nevermind save.
If he will confirm what gear he's running......a 3.23 for example ain't gonna get what I call "good" with a 340. 17 would be about what I'd expect. He could probably bust 20 with a change to a 2.76, for example. But he needs to know what he has now.