Well that depends.  First of all does the car now run?  How much was the
total parts bill?  If the mechanic noticed a problem in the area of
cap/rotor/wires, replacing all 3 at once is not abnormal.  Just because
it wasn't misfiring the night before doesn't mean that there wasn't some
condensation causing problems the next morning.

If your grandmother doesn't take her 850 in for routine maintenance,
then what happened is expected. But if your grandmother's car was
sensitive to damp wires from morning condensation, then it had not
received proper maintenance.  One other thing it could be is a worn
ignition switch.  If it happens again, that would be one thing to check.  
Usually you can get around that by wiggling the key when starting.  That
is not a common problem on Volvos.

My wife's '95 850 once quit on her on the turnpike.  She had it towed to
a garage and called me.  When I got there I got in and drove it home and
it never happened again.  I think in our case is was a computer crash
and leaving it shut off for a while reset the computer.

You might want to consider the fuel pump relay, located under the fuse box
cover, or the fuel pump itself.  The relay is accessed by removing the torx
screws on the cover, but I unfortunately can't remember which relay if the
FP, sorry.  I've repaired a number of  them by resoldering the printed
circuit board where the actual relay attaches to it.

This is a very common problem with Volvo FP relays through the years, and
can really cause a headache because of the intermittent nature of the
failure.

In my experience, the fuel pumps fail after about 110k miles or so, and it
is typical that at the end of it's life it will function properly until the
engine is shut off, whereupon it refuses to function upon restart.

Hope that helps.

Dick