Why would a Fiat suddenly stop its engine?

Hello, my boyfriends car, a Fiat Punto
1.9  from 2001 suddenly stops while
he's driving. He does not know
english and i'm just here trying to help
him even though i understand nothing
abou mechanics so i'll just try
explaining. He was told that the usual
problem that these cars have is the
injection pump so he changed it, but
the car does the same thing an he
said he also put a tester on his car to
see is something is wrong with the
electrical part yet nothing came up.
He works as a mechanic but can't
think of what the problem could be.
He made sure that everything that
could be clogged is unclogged. What
could the problem be? Please help us
:3 now he says that there could still
be a problem in the electric part even
if on the tester it did not come up. Any
ideas?