Problem with friends ICE Vauxhall Zafira

I thought I would post this on behalf of a friend who is having problems with their petrol/gasoline fuelled Vauxhall Zafira.

A few days ago the car service light came on just after the car became very sluggish when pulling away. The car is just creeping along from a standing start which is proving dangerous when pulling out of road ends and roundabouts.

He took it to a garage and it got a full service, new spark plugs, fuel filter, etc. However after just an hour later the issue started again and the service light came back on. He took it back to the garage and they suspect it is a blockage in the exhaust pipe which is worst at low speeds (and low pressure in the exhaust pipe) but seems to be ok when the speed and the pressure builds up. Even now the problem can come and go, with no light and normal pull away speeds but then it comes back on.

The mechanic believes the catalytic converter may have slipped inside the exhaust causing a blockage. He tested this theory by having a colleague rev the engine in neutral and placing his hand a couple of inches away from the exhaust pipe. You can feel that the flow of gases from the exhaust pipe is not constant and seems to splutter a bit.

Any ideas? Is the garage right? Even though the engine management light never came on they ran a full diagnostics and no errors showed up on the system.

They will charge him to do more work but are they looking in the right place to fix it?