What has he tested and how?
As said it could be as simple as the coil.
WHEN IT FAILS TO RUN, DOES IT HAVE FUEL OR SPARK?
If you are asking for help here then relpying with what has been tested and tried is the only way to narrow it down. If not no one will be able to assist you. Glad to help, but can't

If I would have gotten that truck in I would slapped a fuel pressure gauge on it and driven it until it failed. Maybe hooked up a Genisys to it and had another tech ride along (we work fleet, and not flat rate)

GOOD LUCK