Well, bad news.
The inspector said that I definitely *do* have catalytic converters which have been added to the "front tube" indicated by part number 2 on page 10 in the 2003 Pathfinder FSM:
https://www.nicoclub.com/FSM/Pathfinder/2003_Pathfinder/fe.pdf
I believe these "front tubes" also are the part indicated by 12 and described as resonators on this page: https://www.factorynissanparts.net/auto-parts/2003/nissan/pathfinder/le-trim/3-5l-v6-gas-engine/exhaust-system-cat/exhaust-components-scat
The inspector even read off a part number, 445005 on it, which corresponds to a Magnaflow catalytic converter:
https://www.amazon.com/MagnaFlow-445005-Universal-Catalytic-Converter/dp/B00AURR8SK
So, *somehow* these catalytic converters are beneath my passenger compartment. I suspect that my friend had his "muffler" replaced, but actually had the resonators (which are mufflers) replaced. But the people replacing the resonators somehow replaced them with these Magnaflow catalytic converters instead of resonators. They likely did so since the resonators look very much like catalytic converters so they thought they were replacing catalytic converters.
At this point, I am planning to go to an exhaust shop, cut off the extra catalytic converters, and have resonators or straight pipe welded in their place. I could try and go back to the place where my friend got his exhaust done and demand they fix their mistake, but he said it was done a long time ago (God only knows how he got it to pass smog all these years...) and it's likely under no warranty now.
So, is there any advice on how to proceed? I've gone to one exhaust place already. They confirmed what is under my car are catalytic converters, and they've quoted me $300 to replace both of them with resonators and $200 to replace them with straight pipe. I'm going to get another quote from another guy tomorrow morning.
1. Does it make sense to buy straight pipe or resonators myself and provide them for the job so that I only pay for labor?
2. Is it possible to do this job myself? It sounds straight-forward: cut out the existing component, cut pipes that fit, and weld them in. How feasible is this for someone who has never welded?
I am also considering a last ditch effort of going to the original dealer where my friend bought the Pathfinder and asking them if it was sold with these components in place. I'll probably be told the same information: that these are aftermarket catalytic converters, not factory equipment. But if I can avoid having to pay several hundred to get something cut out that was there from the factory, it seems worth a shot.