Our parents would turn in their graves.
After a lifetime of teaching us to help others in need, it would now seem that this is not the wisest of things to do in this day & age.
Hey Les. Were there any independant witnesses to those events ?.
If not, then perhaps it was possible that the driver himself asked for the loan of an extinguisher and put the fire out himself. After all, your driver did advise him at the time that it was not really the right type of extinguisher to use.
Have your bosses asked for several thousand pounds from the drivers insurers. After all, because your driver had used his extinguisher, he was unable to put out a heavy plant fire.