Its a well known issue on that engine. I had a Mk6 GTI (2010 model) that I bought with 55,000km on it and it used to burn oil at about the same rate as your car (I used to check it every 1,500km or so and add a few hundred ml).
Switching to a slightly thicker (when hot) 5w-40 oil seemed to slow the oil burning down a little bit, but it still did it. I used to use Penrite Enviro+ 5w-40.
The issue with the earlier Mk6 GTI's is less so the oil consumption and moreso the dodgy timing chain tensioners which are a ticking time bomb waiting to go off. I heavily suggest your friend factors about $1,600 or so into his bargaining to have the timing chain, timing chain tensioner and other associated parts replaced soon after buying the car. A good number of Mk6 GTI's (and other VW/Audi cars with the same engine) have had timing chain issues resulting in very expensive repairs (eg new engine, or at very least a new head).
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