A friend has a '98 Honda Civic EX, and lately experienced several
instances in which the engine simply shut down while traveling.
The first four times this happened, the engine could be restarted
immediately.  This last time, he had to wait some time (half hour
or an hour) before it would restart.

He brought the car to the dealer who had sold it to him (used),
and they ran engine diagnostics for, according to him, about 20 or
30 minutes.  They found no error codes set, nor any problems reported
during the diagnostics.  The dealer also kept the car and drove it
himself for a week, about 180 miles, without experiencing the
problem.  After my friend got the car back, he drove locally for
about one week without any problem, but then experienced the cutoff
again last week.  This last time, he couldn't restart immediately.

He made a point of noticing this time that when the engine cutoff,
the tachometer dropped to zero after the engine went quiet, thus,
I think, ruling out my initial guess that maybe this was an
intermittent crank sensor failure (if it were, I think that the
tach should have gone to zero first, and then the engine would
cutoff - is that correct?).

My next guess is that it is possibly a fuel pump relay or fuel
injector relay (maybe heat-related) failure, but he doesn't want
to just replace parts without having evidence that the ones on
there now are bad, even though that's exactly what I would do in
this case :-)

He's also not comfortable with doing his own testing on the road
when this happens, e.g., pulling an ignition wire and testing for
spark, and opening the fuel line and checking that gas is being
pumped.

Does this sound like a common failure mode for this car, and if
so, are there any "usual suspects" with respect to its cause?
Apparently, the dealer is reluctant to keep chasing this problem,
but if my friend had some suggestions for troubleshooting he would
be glad to bring the car back and ask that the dealer pursue them.

Thanks for any insight you can offer.