Diesels cost more because...well it's actually that most higher end rigs come with diesel because that's what people paying a lot of money expect.
Reality is there isn't a lot of lifespan difference between gas or diesel. The rest of the rig will typically fall apart before the engine dies. Likewise there isn't a lot of maintenance cost difference (gas has a slight edge but not enough to worry about).
For the same reason diesel rigs are more expensive to maintain...they are higher end with more bells and whistles that can break (bells & whistles that are not part of the diesel).
As others have mentioned, go into it as if the entire cost is a loss...if he's still good with it, proceed.
There is one situation where diesel really starts to make sense...if he's going to spend a lot of time in the mountains...the turbo will mean he doesn't lose a lot of power at altitude and you can get an exhaust brake to help come down steep grades.
