Does this car get lots of short trips ( sub 15 minutes of driving / 5 miles / lots of idling ) or is it parked in a very damp garage? Does this person store the car in winter but starts it every few days and only idles it for a few minutes? I've had customers cars with similar issues before.

The very damp garage one was difficult to diagnose, their brother was also a customer / car guy. He went to the garage and found water flowing from the back wall ( built into a hill side. ) The car was driven semi normally but being parked in a very damp area, as the engine cooled, it drew damp air into the crankcase.