A shared neutral is when more then one circuit uses the same neutral wire instead of its own wire for each circuit. If you are using a shared neutral the two breakers need to be on a common trip breaker or also known as a 2 pole breaker.
A multiwire branch circuit is a branch circuit with a shared neutral. This means there are two or more ungrounded (hot) phase or system conductors with a voltage between them and a shared neutral.
But even if he did turn off the circuit breaker, he could still get shocked if the circuit is part of a multi-wire (shared neutral) circuit which are commonly found in most houses.
