The engine computer monitors the O2 sensor before the cat and the one after the cat and compares the readings. They should vary w/time in a certain way w/respect to each other. It sounds like the one after the cat isn’t doing what the computer thinks it should be doing. A full rich reading could indeed mean the cat is caput. But it could also mean the O2 sensor before the cat is caput, or that both sensors are good, but the engine is running very rich (too much gas) for some other reason. A cat can only do so much. If the input to the cat contains a lot of raw gasoline, the cat won’t be able to catalyze all of it, and a rich post cat O2 sensor reading would result.
Good comments above about things to try. The factory service manual for your car probably has a recommended step by step procedure on what the mechanic should do in this situation. Ask your mechanic to show you that procedure and tell you what the result of each step has been to date.