True, and that's the reason I said "most likely".
Having said that, the oil only comes out when he's boosting hard, which obviously pressurizes his intake system and makes it very likely that there is aleak somewhere.
Check your PVC valve while you are at it.
Another band-aid solution would be venting extra pressure into a catch can.
That's what I do, when I don't boost my car, I don't loose any oil, when I boost it, it all goes into the can.