If it's the stock air box, with everything in place, then he can try running it 1/2 a qt low on oil and see if it stops. If it's an aftermarket breather, then there could be an issue with where the oil blow by goes.
After that, a cylinder leakage test might show something, or at least insure that it's not a scored jug. And there is always the option of modifying the rocker perches to improve drain back. Some people only see them pushing oil out on the highway, and have been known to put a sanitary pad inside the air housing cover to catch it.
My 11 103 pushed enough out to oil the filter down real wet in 5K miles. When I did my stage 2, I modified the rocker perches and it hasn't pushed any in the 6K miles since.