My suggestion is any potential mechanic must be skilled knowledgeable and experienced about German vehicles, particularly Sprinters and equipped with the proper diagnostic equipment. Star Xentry (depending on options $40,000 plus tax and shipping) is the preferred Mercedes-Benz brand diagnostic scanner with annual $3000 online subscription. Xentry's software is updated monthly and the subscription allows the technician to access tech-support and Webnars to keep abreast of the latest information. Reviewing monthly webinars is time consuming.
Take the Sprinter anywhere else and they're going to spin the Big Wheel of Wallet Flushes attempting to diagnose issues on your dime.
You didn't mention model year?
A few items to pay attention depending on MY.
Ask for a DPF regeneration history log. This is an indicator of the exhaust after treatment system's health if DPF regenerations are occurring regularly. (See attached).
Request a print out of the Diesel Particulate Filter's (DPF) "Ash load." This will give an indication of the DPF's remaining Service life.
Many sensors have been upgraded. For example, among others, the pressure differential sensor, back pressure sensor, two NOx sensors, DEF pressure delivery line upgraded with improved materials and the ECU and SCR module's firmware versions upgraded.
Here's a "Cut and Paste" straight outta Mercedes-Benz preowned vehicle checklist. Scrutinize the vehicle carefully and price offer accordingly. There may be a reason he doesn't want to take it to a mechanic.