naffer gurgled happily, sounding much like they were saying:
Only if you can prove that they sheared the plug through negligence, which is extremely unlikely.
What age is the car, what mileage, and have the glow plugs ever been replaced before?
They can and do corrode and seize in place, possibly exacerbated by poor workmanship on replacement last time. It can be effectively impossible to remove them in situ if particularly bad.