I could link a lot of studies - including the new CA Teacher Study - but I've found that these beliefs are often an article of faith with certain crowds. Not going to tilt at windmills today.
Tell your friend there is no soy in a chicken egg. If they have a specific allergen they are...
OK, judging from this thread and this thread I'm going to recommend you focus on the most cost effective complete feed you can find. Forget the no soy, forget the non gmo. Both add substantially to the price of your feed without any particularl benefit to your birds.
Purina Flock Raiser...
Define "best". Do you want least effort, best nutrition for your girls, cost effective [not that one, based on your question], something else?
By eliminating soy, you are already making it difficult - not impossible - but difficult to make a cost effective, nutritionally superior, feed -...