Make sure he gets a 'primer' on starting the diesel... wait for glow plug light, wait 3 seconds, whatever. It's a different process- reading about it in the manual while sitting in a parking lot is no fun.

Put a post-it note somewhere about filling up with DIESEL..... not regular gasoline. A late-night stop when tired/distracted can be costly/ embarrassing.

Drive the non-interstate to get familiar, check the tires/hubs after a while before going for high speed travel (check air pressures before).

If he's buying it from "one of us," I'd hope it's been cared for and offers little risk.

Road trip!