It's not about the crankcase gasses - they're still going into the intake pipe same as always - follow the ProVent 200 plumbing and you'll note that it's outlet connects to the engines air intake at the point that used to be the DPV.
The purpose of the ProVent is for the oil separation. It removes the engine oil out of the crank case gases. This engine oil can either then be drained to waste (or hopefully sent for recycling) or tapped in to return to the sump. The reasoning for this is to prevent it entering the air intake system, coating it in oil (especially the intercooler which reduces it's effect) and unnecessarily burning oil through the engine.
Why? Why would you clean something that doesn't need cleaning? If you prevent the oil from entering into the intercooler in the first place then you won't need to clean it any more. As only air will be going in, which has already been filtered through the air cleaner before the turbocharger and intercooler, there's now no longer anything in the intercooler to clean. So it doesn't seem like a necessary or good move any more - this is the point of the ProVent.
It's perhaps possible that it wasn't installed correctly - perhaps the inlet and outlet connections were connected backwards, or the Td5's original DCV was left in place incorrectly when it needs to be removed to enable the ProVent 200's PCV to be connected.
Or perhaps your friend was sold a knock off counterfeit version. Sadly these are very common and their quality, performance and operational functions are not right. I note you said that your friend binned it - the genuine units are rather expensive so I'm surprised someone would just throw it away over reselling - this makes me wonder if it was therefore a knock off.
Or maybe the unit your friend had was genuine but was faulty and should have been returned for a warranty claim.
Or maybe the filter was full, or there was a blockage.
So many possibilities that a single case can hardly be considered sufficient to write the product off...
Now in saying all of the above, I still have a theory that the ProVent 200's PCV builds up more pressure than the Td5's original DCV supplied by Land Rover. I need to dig out a DCV and do some testing on this. Perhaps the balance might be in gutting (or keeping open) the ProVent's PCV so that the DCV can be refitted and still function properly without the two causing each other issues - at this point though it's possible as effective to use a cheaper catch can with a PCV built in.
ProVent Catch Can Oil Mist Separator Explained - YouTube

