I wont advise you on this as im not sure about the legalities, however it is very common to totally screw the engine when the cam belt breaks, im just a little confused as to why he didn't notice damage to the pistons when he had the head off. Incidentally, it is impossible for the pistons to clash together, with a cam belt breaking the pistons can hit the valves, this is normally very evident when the head comes off, the pistons almost certainly hit the valves, that is probably why you needed a new head.