It does sound like your "mechanic" was trying to swindle you. Which is why I asked for clarification. The catalytic converters are the most expensive exhaust component there is, and generally they last the life of the vehicle. Which is why I'm suspicious when somebody nonchalantly says, "Oh gee, looks like you need a new one. Should get a dealership part too."
Since Turdle hasn't mentioned his own thread I'll go ahead and do it. These three threads may help you or other future readers:
Link 1
Link 2
Link 3