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Cheap nasty sprockets would be my guess. They don't usually break off until they are totally hooked and worn. If they snapped off and the remaining teeth have a reasonable profile I would make sure he doesn't fit that brand of hardware again....and tell us the make so we can avoid it.
Definately sounds like way over due replacement. I wouldn't do anything less than replacing both sprockets and chain (along with a talk with your friend about pre-ride inspections and preventative maintenance.