PugsAreLife

Hey Scotty,
2001 toyota corolla ce 188k miles. Been watching all the videos for codes p0171 and p0420. The person I am trying to help has these codes. When I looked at it had oil barely touching the dipstick and was very black. Replaced oil and filter. When I looked at the spark plugs they had oil on them and the insulators broken on all. The car is burning a lot of oil. I know for sure it has driven about 100 miles and after I changed the oil it dropped 3/4 a quart. The whole cloth piece on the hood is soaked in oil so everything under the hood has oil residue. I did not see oil puddles in the driveway. Radiator also was just water so I also drained and filled with coolant. I cleaned the MAF sensor and the air filter was clean. Next I was going to see if there was an air leak for the lean code before replacing the O2 b1s1 and check pcv. Bluedriver says replace cat and/or o2 sensor to fix p0420. I have not done a compression test or test for head gasket leak. I don't want this person to spend endless amount of money trying to fix. It doesn't overheat and starts okay with the crappy plugs it has. What do you suggest? Thank you