When the timing belt is renewed I would always recommend that all the idler and tensioner pulleys be renewed at the same time, together with a brand new water pump. The reason for this is that although it costs more than just replacing the belt, if the old water pump is left in situ, if it seizes, or if the old pulleys fail, then the new timing belt will snap or be thrown, with the catastrophic consequences that you describe.
Many owners choose the cheapest price for renewing the belt, which can work out a lot more expensive in the long run if the additional work described above is not carried out at the same time.
ChicksFan
August 2013