it would be helpful to get the actual code number it is throwing vs being told " driver's side upstream" issue to start with.

From what I gather you're talking about the driver's side portion of the Y-pipe that starts down-stream of both of the cats and merges into the single exhaust collector, right?

Regardless, clearing the codes does nothing if the problem is still there. Once the system's initialize you'll get the light back on. I see a couple of scenarios of what could have happened ...maybe.. IF what he says about the code being for the upstream sensor is correct I don't see how that could be unless he also unplugged that sensor and possibly forgot to plug it in while first testing the exhaust when he finished up.

I guess it could also be possible that the sensor and or wiring were on the verge of going out and failed in the removal process. If I had to guess getting those exhaust bolts wasn't the easiest task and there was probably some impact wrench action happening... so it isn't out of the realm of possibilities that the sensor vibrated itself to death (not necessarily his fault if you ask me).

I have also seen catalytic converters meet their death during related exhaust fixes/removal with the same said vibration. I think your best bet is getting a clear code number to start with