I don't think Insurance will cover you friend's negligence.
Potentially he could also be looking at a new engine. Diesel is in itself a lubricant and as such is used to lubricate things like the high pressure fuel pump, whereas petrol will act as a cleaner and strip all lube. You can even get problems with things like fuel lines if they use a rubber that is incompatible with petrol. You can read a lot of examples of people petrol fuelling a diesel years ago, but the modern common rail engines just aren't that tolerant.
See here:
http://www.telegraph.co.uk/motoring/...7/mfdies27.xml
And buy him one of these :
http://www.netparts.co.uk/store/inde...FQQdEgodqHQm2Q