I'm not too clever with ET's and stuff
But your friend will need 5x100 (mk4 golf) to 5x112 wheel adapters..
These will probably have to be bolt on adapters as hes changing the bolt pattern thing from 100-112
So will probably be 18mm spacers and above (as the wheel bolts will need enough material to thread into).
He's got staggered set of wheels, and with the spacers/adapters he will have abit of poke, so MAY need to roll the arches or fit tyres with a stretch to stay legal. (I think we fail mots if you can see the tread of the tyre sticking out of the arch when you look down onto the tyre from the top)
Also when fitting adapters/spacers to the front axle, good to have a mate on the foot brake or you'll be there all day lol
Hope this helps