welcome to the forum, and so glad to hear you are reaching out to share Jesus with your friend.
The premise of this statement says that God is a creature. The Bible says that God is the eternal uncreated first cause.
Your friend seems to have a view of god (I will use little g), is a projection of his mind, an idea. He then asks why should this projection of his mind have rationality. The problem is if your friend is simply matter/energy, his mind is simply neurons firing which doesn’t lead to truth, and it undermines his own rationality.
Lennox, one of my favorite writers and speakers says this:
Do atheists have faith?
This confusion about the nature of faith leads many people to another serious error: thinking that neither atheism nor science involves faith. Yet, the irony is that atheism is a belief system and science cannot do without faith.
Physicist Paul Davies says that the right scientific attitude is essentially theological: “Science can proceed only if the scientist adopts an essentially theological worldview”. He points out that “even the most atheistic scientist accepts as an act of faith [emphasis mine] … a law-like order in nature that is at least in part comprehensible to us”.
Albert Einstein famously said:
“… science can only be created by those who are thoroughly imbued with the aspiration towards truth and understanding. This source of feeling, however, springs from the sphere of religion. To this there also belongs the faith in the possibility that the regulations valid for the world of existence are rational, that is, comprehensible to reason. I cannot conceive a genuine man of science without that profound faith [emphasis mine]. The situation may be expressed by an image: science without religion is lame, religion without science is blind.”
Einstein evidently did not suffer from Dawkins’ delusion that all faith is blind faith. Einstein speaks of the “profound faith” of the scientist in the rational intelligibility of the universe. He could not imagine a scientist without it. For instance, scientists believe (= have faith) that electrons electrons exist and that Einstein’s theory of relativity holds because both are supported by evidence based on observation and experimentation.
My lecturer in quantum mechanics at Cambridge, Professor Sir John Polkinghorne, wrote, “Science does not explain the mathematical intelligibility of the physical world, for it is part of science’s founding faith [notice his explicit use of the word] that this is so…” for the simple reason that you cannot begin to do physics without believing in that intelligibility. On what evidence, therefore, do scientists base their faith in the rational intelligibility of the universe, which allows them to do science? The first thing to notice is that human reason did not create the universe. This point is so obvious that at first it might seem trivial; but it is, in fact, of fundamental importance when we come to assess the validity of our cognitive faculties. Not only did we not create the universe, but we did not create our own powers of reason either. We can develop our rational faculties by use; but we did not originate them. How can it be, then, that what goes on in our tiny heads can give us anything near a true account of reality? How can it be that a mathematical equation thought up in the mind of a mathematician can correspond to the workings of the universe?
It was this very question that led Einstein to say, “The most incomprehensible thing about the world is that it is comprehensible”. Similarly the Nobel Prize winning physicist Eugene Wigner once wrote a famous paper entitled, “The unreasonable effectiveness of mathematics in the natural sciences”. But it is only unreasonable from an atheistic perspective. From the biblical point of view, it resonates perfectly with the statements: “In the beginning was the Word … and the Word was God … All things came to be through him” (John 1 v 1, 3).
Sometimes, when in conversation with my fellow scientists, I ask them
“What do you do science with?”
“My mind,” say some, and others, who hold the view that the mind is the brain, say, “My brain”.
“Tell me about your brain? How does it come to exist?”
“By means of natural, mindless, unguided processes.”
“Why, then, do you trust it?” I ask. “If you thought that your computer was the end product of mindless unguided processes, would you trust it?”
“Not in a million years,” comes the reply.
“You clearly have a problem then.”
After a pregnant pause they sometimes ask me where I got this argument—they find the answer rather surprising: Charles Darwin. He wrote: …with me the horrid doubt always arises whether the convictions of man’s mind, which has been developed from the mind of the lower animals, are of any value or at all trustworthy.
For thought is replaced by electrochemical neural events. Two such events cannot confront each other in rational discourse. They are neither right nor wrong—they simply happen. The world of rational discourse disappears into the absurd chatter of firing synapses. Quite frankly that can’t be right and none of us believe it to be so.
Lennox, John. Can Science Explain Everything? . The Good Book Company. Kindle Edition.
Maybe buy your friend a copy of this book? It’s brilliant and very understandable, and builds a very good case, and in the second last chapter shares the Gospel in ‘conversation on a train’. I keep a copy in my car glove box in case I can give it to someone as a follow up to a conversation.
Lennox has written two other goods books which are more in-depth which your friend could read after. ‘Gunning for God: Why the New Atheists have missed the mark’; and 'God’s Undertaker: Has Science Buried God".
Hopefully this is a helpful start…