I'd run an intake, straight exhaust, and as stated, a custom written SCT. The SCT is flash tuning device that can have custom files written to it and than those can be loaded onto your friend rig via the diagnostic port. Depending on the tuner you select, the tunes can go from 50-80 dollars a tune.
Intake and exhaust just open up the trucks breathing. It's hard to get anything over 1-3mpg from any mods without a solid tune going on the truck, that's where the SCT will do wonders.
I would recommend looking into
Innovative Diesel tunes. I have run their tunes for as long as I've owned my truck and won't ever look back! From mild to wild, Eric writes one amazing tune, specific for your truck and parts on it!