I do understand where you are coming from and I was only a one man repair business, just about managing to provide for a wife and five children, so not a wealthy garage proprietor exactly and that was without two ex wives.
So my advice is for your friend to look at the cost of the diagnostic tool I mentioned which is probably much less than he has already spent with other peoples "parts canon" which my advice is aimed at preventing. Many on the Fiat Forum run those engines on campers so happy to offer advice.
The reason I mentioned the Snap On tool is that it is very good and some like me with contacts in the Motor trade are able to access one without the major investment, I was hardly likely to suggest the expense of buying that equipment as it needs to be in daily use to justify the cost.
So start with the basics, compression, timing, fuel.
If you are happy you have four good compressions, the valve timing is correct, the rev counter is working so your crank sensor is also, cam shaft sensor checked? The immobiliser light comes on and goes off before engine starts to crank? You have a good supply of clean diesel getting to the high pressure pump? Can you measure for voltage at the high pressure pump regulator and cut off?
Did you say you were getting some fuel from common rail feed, was that steady when cranking or what just dribbled out?
If you feel there is some fuel to injectors then you can do what is known as a "leak off test", this is where you measure the fuel from each individual injectors rubber return pipe, as sometimes one will fail and return a much higher volume than the other three injectors , telling you it is faulty, if that is the case the system will not build enough pressure for the ECU to tell the injectors to open and fire the engine.
You can Google leak off test to explain it.
