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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.
I've seen missing teeth in the distant past. That suggests the other sprocket is also knackered (that's a technical term indicating excess wear!) and it deserves close inspection, before spoiling a new chain.