Hello Mark,
I've no experience of the Vauxhall/Renault 1.6cdi engine in these brands, But I've had issues with this unit in Mercedes Vito vans at low miles 30k or so.
Can you get an inlet or outlet hose off to check for end float of the impeller?
On the Vito's I've done the turbo whistles like crazy but not had any oil lose.

Our Vito's are on our fleet so problems get flagged up as routine, But maybe the owner of this Vauxhall has been ignoring the whistle to the point where there thrust washers have worn so much it is allowing oil past the bearing!!??

HTH
Ian

Please ignore this reply. I thought it was referring to the 1.6 Cdi Diesel not a Petrol unit
Oops
But still check end float