Glowplugs are not generally required for cold starting in ambient temperatures above about 10C.
Assuming your father left the car in the supermarket for only an hour or two, it must have been warm when he tried to restart it-so no glow plug cycle would be required.
Once started the car will be perfectly driveable with a failed glow plug although a bit clattery and possibly smokey until the engine starts to warm up.
It is very difficult to see how an exhaust component could stop an engine from starting. There are a variety of reasons why a diesel engine would fail to start, ranging from a failed crank sensor (cheap) to a failed high pressure fuel pump (expensive). It does sound like a bunch of incompetents are drying diagnosis by substitution.
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