Dis that sounds like there was something not being mentioned by your friend, as to go to jail for such an offence would require it to be dangerous driving or similar, so I'm going to go out on a limb and say that there was much more to it than the guy just being 6mph over the limit at the time of the accident (it's rare for anyone to go to jail for a simple accident that results in a death, usually there will be other factors such as drinking, drugs, unsafe vehicle, being banned at the time of the accident).
Don't get me wrong, he wasn't my friend.
I think the reason for it is in the last few months before the crash there were several people run over and killed on the same stretch of road. It was originally a 60mph limit dropping to 30. They then lowered it to 40 into 30 as a result of an elderly man and young boy dying. Basically the judge made an example of him saying he hadn't learned his lesson from the previous, well publicised, deaths on the road.
http://www.chroniclelive.co.uk/news/north-east-news/speed-limit-cut-tragedy-road-1505180
Article sums up some of the previous accidents.