my friend has the old chaos mortals book but knows some rules from the new book does he use the new book rules or old
In friendly games its up to his opponent....In Tournament games, you must use the newest book. And I personally would say use the old book or new one, but don't mix them...
I agree each edition has separate benefits and restrictions, so I always played with my friends that you play with old or new, your choice but choose one or the other, or else you get all the benefits with no negatives, make it a tad unbalanced.