He will not see a leak as the oil is going into his raw water and out the Exhaust. While at the same time some of the raw seawater is coming into the oil pan. (He needs to look and validate the theory by observing the Exhaust to see if there is oil in the cooling water.

I had a Perkins that did that and it was the oil heat exchanger that was the culprit. Once I fixed that at a local Radiator re-build shop there was never a problem (since 1996).