You have to do something pretty bad not to be under warranty on a less than three yo vauxhall. They come with a three year warranty and the first owner gets 100,000 miles.
One of the sensors has failed or detected an out of spec error. The chances of the cat having failed is low but it could fail if the other fault isn't fixed. One way is for there to be too much unburnt fuel in the exhaust and that can cause the cat to go pop and you get lots of expensive platinum coated ceramic bits falling out the tail pipe. Other than poisoning it by putting leaded fuel in I can't think of another way to kill a cat.
There are plenty of ways of killing the sensors, like silicates in the petrol (ask Tesco about that).
What's he going to do when the MOT is up scrap the car?