I feel sorry for Billy. He goes in for a game of CS and his friends have dirtied up his car in public. I don't think you should ever do that, even if it is washable, but if you really wanted to do it, do it in his driveway or something, not in public.
If I were you guys I'd destroy the video tape, all of you admit your guilt and buy him a six pack and let him do his thing. While he's doing that all of you get the bucket and hose out and wash his car, inside and out, tyres and all, vacuum, glass cleaner, the whole deal. A gift certificate for someone else to clean it is too easy. Apologize again and hopefully he'll forgive you for it.