Its on a sticker on the inside of the service book front cover.If the owner has the original service book (its either blue and white front cover or pinky red colour for 2004) it will indeed be inside the front cover, at the bottom of the stuck in label. He could be confusing it with the warranty and maintenance guide which has a similar appearance.
Failing that, a trip to his local dealer, along with the V5, ID and his debit card will get him the code for the original unit that was in the car from new.
Oh! looks like I'll have to pop over with my radio keys tomorrow and have a look for that! However, I swear my old Megane was all numbers...It will be 4 digit number and as everyone has said, it will be in the front of his service book on a sticker with the chassis number, engine number etc