I'm assuming nobody was hurt.. I'm also assuming you gave your details to the person whose car you damaged. If you haven't done that, you must do so, or tell the police if you can't find them.
Then get a claim form from your insurer. Fill it in, giving as much detail as possible of you, the car you were driving and the 3rd party car you've damaged.
The 3rd party will need to get an estimate of repair costs for their vehicle. If they have comprehensive cover, their insurer will fight their case with your insurer.
As CovKid says, your cover will probably only be 3rd party. This means that the C2 will get repaired or replaced, but your mate won't get paid out for his car. You may be lucky, but I doubt it.