24V Code list....
Using TTMT's most excellent cool code list, it looks like yet another VP44 joined the great rebuildable stock pool

The codes on your buddy's truck translate as follows:
P0251 (M) Fuel Inj. Pump Mech. Failure Fuel
Valve Feedback Circuit
Problem sensed with fuel circuit internal to fuel injection
pump.
P0253 (M) Fuel Injection Pump Fuel Valve
Open Circuit
Problem sensed with fuel circuit internal to fuel injection
pump.
P1688 (M) Internal Fuel Injection Pump
Controller Failure
Internal problem within the fuel injection pump. Low
power, engine derated, or engine stops.
P1689 (M) No Communication Between ECM
and Injection Pump Module
Data link circuit failure between ECM and fuel injection
pump. Low power, engine derated, or engine stops.
P1690 (M) Fuel Injection Pump CKP Sensor
Does Not Agree With ECM CKP
Sensor
Problem in fuel sync signal. Possible injection pump
timing problem. Low power, engine derated, or engine
stops.
When mine died all I got was:
"P1693 (M) DTC Detected in PCM/ECM or DTC
Detected in ECM
A 9Companion DTC9 was set in both the ECM and PCM. " Which means a ton of nothing...

I ended up having a local Diesel shop troubleshoot it, as I didn't want to gamble on a new VP44 w/out a clear code.
the list is not the FAQ section at the head of this section. It's URL is:
https://www.turbodieselregister.com/forums/showthread.php?t=9663
Make sure your buddy adds a Fuelpressure gauge and heeds it's warning, Good Luck, Moose