Did this unfortunate occurrence happen in one ride? ie, not there when he set off, but lodged and worn through by the end of the ride? If that is the case then indeed unfortunate. If not, how long does he think it was there, wearing away, undiscovered? Seems to me that your friend’s predicament does not make a firm case for not fitting radiator guards.Until a stone gets caught between it and the radiator and wears a hole in the radiator. Happened to a friend of mine on a fireblade.
Alan R