*Air

*Good fuel

*compression

These are the only three things that one of these diesels needs to run. Without one, it cant run.

A good starter (that turns over fast) and a well charged battery help a lot too

Make sure there is NO air in the fuel lines, and make sure the GP's are getting voltage. Fresh oil is a have to. Then crank her over...

It takes quite some time to bleed the air out of the Injection pump, so be pacient. You might want to check for fuel at the injectors. This is accomplished my loosening the hard lines at the injectors and cranking it over while watching for fuel.

If he just changed the filters I am thinking that he never bled all the air out of the lines. But, if it wont start after 10 minutes of total crank time (15 seconds max at a time, 30 seconds at least in between) then you have a problem.