Re: 7.3L 1999 Ford F550 Trouble Starting
First, I'd get your boss to visit a shop that has some diesel experience.
For no start/hard start concerns, we will generally set up the scan tool to monitor what I have come to call the "no start PIDs" before I attempt to start the vehice for the first time. These parameters are V_REF (PCM reference voltage), RPM (engine RPM), ICP_V (injection control pressure measured in volts - an actual value), ICP_P (injection control pressure in PSI - *sometimes* this could be a substituted value if the PCM doesn't like something - it can take some experience to tell), IPR (injection pressure regulator duty cycle). Having this test performed should give the tech enough info to decide his next step.
Some idea of how many miles since last oil change and how many miles total might give a clearer mental picture of the truck.
If asked for a SWAG, I'd think that the high pressure reservoir is leaking down or the batteries get weak enough to lower cranking rpm.
In the end, nothing compares with a logical, step by step diagnostic process to reduce repair time and minimize needless parts replacement.