Cat converter or not? 2010

[Crazygirl]

My daughter recently bought a 2010 Aveo 5 LT, and from the beginning the check engine light has come on. It's throwing a P0420 code. She took it back to the dealer twice to be looked at, and all they did was replace a hose on top of the engine that they said had the mass airflow sensor in it. They swore the car is fine, but after the second visit, I got the car a mile from the dealer and the light came on.
We took it to a couple of different mechanics, and they both said they believed it was the catalytic converter. The second mechanic, a very reputable company, said her car actually has 2 cat converters. However, because the part they assume is dysfunctional is a special order, they replaced one of the O2 sensors (I believe it was the rear sensor), and advised her to see if that helps. That part was covered under the extended warranty she bought; the cat converter is not covered, not surprisingly.
She drove the car for several days after getting it back, and all seemed well. No performance issues, good gas mileage, just what you'd expect. Yesterday morning, she finally filled the gas tank, and 15 minutes later, while driving to work, the light came on again. I was hoping maybe it's a loose gas cap and advised her to make sure it was on secure.
Today, after work, she took it to Autozone, who read the codes, and it's throwing the P0420-again.
Since she drove the car for a few days with no problem, what are the chances that the cat converter really is bad, and it just seemed fine for a while because the O2 sensor was replaced? Should I just take it back and tell them to go ahead and replace the CC, to the tune of about $1000, parts and labor? If it weren't for the fact that we would forfeit the $4000 down payment we already spent, I'd tell her to take the stupid thing back and tell the dealership to park that thing promptly up their a**es. Thanks!

[aleekat]

Most cars have a 10yr warranty directly on the cat. Check owner's manual.