Weird...definitely a build up of pressure in the engine to make it leak if the oil cap is on, but not leak if there is a way for the pressure to escape.
Almost sounds like a plugged PCV (in a normal engine) problem, but he says he checked the line and it's not clogged. Then again, it could be clogged inside where he can't see/get to. I don't know anything about diesels, so I can't help him there.
Just sounds like the exhaust gasses aren't escaping properly and thus building pressure into the engine and making the oil squirt out wherever it can find a place to do so. Bad thing is, even after the pressure issue is fixed, he will probably have to replace whatever seals the oil squirted out of because they are now probably permanently damaged.