First point, G is 89/90, by this time all Golfs ran unleaded with no mods needed. From memory all Golfs imported after 1987 would run on unleaded. My first Golf which was an 86 did need leaded fuel.

Ordinary Golfs like the Match of this age will run quite happily on 95 octane fuel, in fact they will also run on 91 Octane with a slight loss of performance, something you will never encounter since its never been available in the UK. If you put super unleaded in the only it will do no good whatsoever but other than harming your friends wallett.

Golf GTI's of this period were said by VW to need 98 Octane unleaded but could run 95 Octane with a slight loss of performance. I did 113,000 miles in an 1989 car over 7 years and when I originally bought it checked the mpg several times. The grade made no difference and there was no noticable difference in performance whichever fuel I used, even when towing. I would say that at least 112,000 of those miles were done on 95 octance and when i sold it the car was was running perfectly. The car finally died last November aged 24 1/2 years. God knows how many miles it had done by then, last time I actually spoke to the owner it was 11 years old and had 180,000 miles on it and was still running perfectly.

Tell your friend to save their money.