In accordance with previous posts steer clear of an SDI.
If your friend is after a diesel golf please go for the 1.9TDI, I had one a couple of cars ago and whilst they were a truly fabulous drive VW reliability isnt what it was I'm afraid (in my experinence) went back to dealership under VW warranty 5 times (must say however that VW cust service was top notch!). I wouldnt buy another VW for a while im afraid desipte them being great drives and having great customer service, had the bad experience with reliability.
Had the advice of a family friend who is one of the most senior mechanics in the UK when I was shopping around for VW's and he said if you want a golf buy anything but the SDI, it is woefully underpowered, not just in a boy racer beating people from the lights way, in a literally struggling to pass HGVs on the motorway kind of way, apparently attempting to pass on a single lane carriageway is a terrifying experience due to the engine's pathetic amounts of torque.
SDI resale values are abysmal so for 100k on the clock you should be paying near enough half of the quoted price!! At that milage and spec your friend would be the final owner so 3k is a lot of money to run into the ground!
If you are genuine in who the car is for, buy something small, cheap and Japanese, preferably Toyota, Honda or Nissan (controverisal last one I know but had 2 nissans and was amazed by their reliability, just make sure you stick to the smallish petrol engines!)