First off I hope the guy recovers quickly. Very lucky escape, although I doubt he's feeling very lucky right now.
Second, this is a lesson I learnt myself the hard way. First group ride and went sliding down the middle of the road on my arse. In the middle of the group (alone at the time), was pushing myself and then decided to ease off as I realised I was pushing too hard... came out of a nasty gravelly s-bend and then lost it on the the next corner. Apparently a porsche had done the same the day before but went through a hedge on the other side of the road instead. It's known as a damn nasty corner as it tightens up suddenly and tends to have crap on the road (wish I'd known that). Luckily I walked away and got a lesson in mechanics fixing the bike up (would have been a write off otherwise and didn't fancy loosing my no claims bonus).
I won't be making the same mistake twice!