Overshifting = forcing the shifter passed the top gear. If he's not used to riding with gears and was doing something dumb like riding big ring to big sprocket he and getting the chain rubbing on the cage, he may have tried to shift it further and hence broken it
EDIT: Oh, he's ridden gears before.. in which case.. unless he is just dumb :) it's a warranty
Overshifting = forcing the shifter passed the top gear. If he's not used to riding with gears and was doing something dumb like riding big ring to big sprocket he and getting the chain rubbing on the cage, he may have tried to shift it further and hence broken it
EDIT: Oh, he's ridden gears before.. in which case.. unless he is just dumb :) it's a warranty