I’m afraid he didn’t pay heed to the label right next to the oil fill. That little label that says use only the proper oils in this engine or it will cause emission damage.
The particulate filter is probably wrecked from the ash content in regular oil. It is the one under the vehicle about under the passenger’s seat. But before you condemn it double check that the differential pressure sensor across the filter is not clogged up. There are 2 tubes coming out of the filter that go to rubber hoses that go to a sensor underneath the car. Make sure the filter ports are clear, the hoses are clear, and the sensor ports are clear before shelling out $$$$ for the new DPF.
The oil filter housing can be resealed, there is a Dorman kit on Amazon
https://www.amazon.com/Dorman-926-365-Filter-Housing-Mercedes-Benz/dp/B08K1LXKWQ
You get both the middle of the neck gasket as well as the flange to head gasket.
OM642 oil filter gasket
EGT probe is in the base of the turbocharger. You may not be able to get it out without pulling the turbo
OM642 VNT Turbo actuator issues
And the mother of all threads on the oil cooler seal replacement on the 642 (step 1 is pull the turbo so you can swap the EGT probe while it’s out)
A journey into the the infamous Om642 oil cooler leak!
I have had tons of experience with mine, fire away with the questions and I will try to help. I fixed mine up and handed it down to my son and now he’s driving it. He has about 230 k on his too.