Infiniti - G35 :: Fixing SES P0430 Code?

I recently purchased a used, high mileage 2004 Infiniti G35x (AWD) 145K miles from a reputable new car dealer here in my town. The car was impeccably maintained and clean inside and out. Just a week or two after the purchase my SES light came on revealing engine Code: P0430 (catalyst efficiency below threshold bank 2) pointing to a new catalytic converter in my future. Infiniti recommends premium unleaded for this vehicle. I found out my son filled her up with mid-grade unleaded. Is there any chance this could have triggered the SES light to come on? (Previous owner states he always used premium unleaded in the vehicle since ownership.) I also double checked the gas cap to make sure it was on tight. I find it extremely odd that the SES light would come on so shortly after the purchase. I mean what are the odds? Don't dealers run a diagnostic scan of their trade-ins to check history and for potential problems prior to selling their used cars? I test drove this car numerous times without any indication of a problem and right on the heels of the purchase it passed state emissions... now just a few days later, the SES light comes on. Why wouldn't emissions pick that up? I have reset the SES light (code) and filled her up with premium unleaded. Now it's a waiting game to see if the light comes on again. If the light reappears, I plan on taking it back to the dealer to work out some kind of "reasonable solution" to repair costs. I'll find out just how willing he is to stand behind his cars. I realize that this is a high mileage car, and I hardly expect perfection, but I didn't see this coming. What to do... what to do... By the way, I checked all the fluid levels ... no discoloration in the oil (fresh, golden brown)... no discoloration in the coolant... transmission fluid, good... all other levels are good. Car runs fine, no rough idle in cold...