The person you were talking to has no idea how Greek works. Let me explain how the greek noun system works, then I will explain why they have a false understanding of John 1:1c.
In English we tend to order our sentences in such a way that the subject of the sentence comes first, and the direct or indirect object of the verb comes after the verb. So for example: John (subject) kicked the ball (direct object). Generally, the way a noun is being used is dictated by its position in the sentence.
Greek doesn’t work that way. In Greek, the sequence of the wording has quite a bit of flexibility, and word order often has less to do with which noun is functioning in what way. As a matter of fact, Greek often uses word order to show emphasis, so saying something first in the sentence even though the sequence or thought flow doesn’t quite follow that order, is the author’s way of saying this is really important. Paul does this for example in Ephesians 2:8 when he says For by grace you have been saved through faith. In English we would normally say, “You (subject) have been saved (passive verb) by grace (the indirect object performing the action) through faith (a prepositional phrase explaining the agent by which grace is working).” Paul however, flips the word order of the sentence to place emphasis on the grace.
So as demonstrated, word order doesn’t dictate noun function. So how do the Greeks determine which noun is acting in what way? They use case endings which are added to the root of the noun. So every noun has a root that carries the meaning of the word. In the case of John 1:1, it would be Theo (God). We add a case ending to the noun root to tell us how the noun is functioning in the sentence. When the noun is functioning as the subject of the sentence, we say it is in the nominative case. To do so, we add a sigma to the end of the noun root, which gives us the word Theos. If we want the word to function as a direct object, we say that the noun root is taking the accusative case, and add a nu to the end of the word, giving us the word Theon. The word means the same thing, it is just operating as a different grammatical function.