how is your brother? did the chain come off either end of the cassette or simply slide over them? I would check the chain/cassette for wear. it is possible that the chain is just a bit too narrow for the cassette
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“One morning you wake up, the girl is gone, the bikes are gone, all that's left behind is a pair of old tires and a tube of tubular glue, all squeezed out"
Sugar "Kane" Kowalczyk