Changing speeds a bit...

I know with some pumps, you must have enough head for them to work properly (or properly, you shouldn't be trying to raise the fluid higher than the pump's capacity). So...

If there was a column of air between the fuel tank and the engine (i.e. when you change fuel lines), would the amount of fuel in the tank make it impossible to purge the air?

I have this friend (totally hypothetical, of course) who has about 1/4 tank of diesel, has gone through the charge of two batteries and about 1000 primer strokes, and still has a car in his driveway that doesn't start. Do I... I mean.. does this guy need to get more fuel in his tank or is it likely something else?