Having the knowledge of good and evil, as God does, would have equipped Eve and Adam to fully understand Gods first law; thou shalt not eat....Making then the serpent's leading to do just that of none effect. When they had no comprehension of good nor evil, they had no conscious ability to make an informed free choice to either obey, or disobey. Whereas the Serpent was far wiser, and let into the garden to exploit that very thing.
That is why it is later written sin was in the world even then. Because sin is not imputed where there is no law. Sin came into the world the moment God uttered to Eve and Adam, thou shalt not.... While Saul/Paul tells us sin was in the world before the law of God was given.
That is because when sin is not imputed where there is no law, the law must have its defined within it non-compliance factors. I.E. punishment for transgressing the law. And all this was created by God the giver of the law is also that one which created the imputation (Impute= attribute, ascribe] for its transgression.
Oh, the outrage, so many may imagine themselves to feel when reading that. However, it is Biblical. The Book of Isaiah 45.
It's the woman's fault! Eve succumbed to the serpent and his temptation. Ah, how we so overlook the Biblical fact that God blamed Adam for sin entering the world. (Which is interesting when Saul/Paul said sin existed in the world before the law. A man informed by the ascended Christ to bring further revelations to the world. That too is interesting. And perhaps a topic for a different thread. )
God is the creator of all that is. Not even Satan is able to fully do whatever he wishes without Gods allowance. As we learned when we read of the trials of Job.
The question then becomes, why did our omniscient Father not forgive the first innocents for a mistake that was not their fault, nor choice? Not wilful choice due to having no conscious awareness of good nor evil so as to make an informed decision to either obey or disobey.
The first innocent guileless couple He created. And then when in that paradise garden, where a tree that would damn the world due to the fruit it bore if consumed, would not only make the eater thereof like God, then and only then knowing right from wrong, good and evil, so as to then be conscious of those dual components that are enmeshed in this world of light and darkness, was consumed, and due to a wise serpent with awareness of that tree being allowed entry into a place overseen by omnipresent father.
Only after eating did the first people become conscious of right and wrong. And only then did they recognize they were naked and for some reason not present in their psyche prior, were ashamed of that God given flesh and bone created in the image and likeness of their Father.
It's a mystery! Or is it? Without that happening in the beginning, we would not be aware of the dual energies operating in this world; good and evil. And here, where the adversary, adverse, of God and His great goodness, exists, while also being allowed to re-enter Heaven from which he was once cast out, and so as to confer with God, roams about , walks to and fro, like a hungry lion ......seeking souls to devour.
Without that which happened in the garden, we would not know what God is. We would not know what evil is. And we would not then, because we are now conscious of good and evil, be able to make the choice. To seek Good/God. Or to die in the evil/sins. And what are the sins? And why do they exist?
The Book of Isaiah chapter48
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I'll be brief in my explanation. Rather than think the brother was wrong in their sharing their Biblical perspective due to their relationship with God and His word, perhaps take time to consider that you may benefit from someone else and their perspective. 
