Carabiners are generally quite sturdy mechanisms. I have a few that are more than sixty years old that still work quite well, although I do not place them in critical locations. More modern biners are lighter for equivalent strengths and can withstand moderate abuse (short of pounding them with a sledge hammer, for instance). Improperly rigged, it is possible for the gate to be forced open which could lead to failure of whatever system the biner was employed, but the biner itself would not be "broken."

More details would be useful. I suspect the victim in this case did not have a strong technical climbing background - venturing alone into a mine shaft is a very bad idea - something never recommended.
Geezer in Chief