We can distinguish between a thing’s essence and existence - namely, what it is and that it is. There are things in which essence and existence are not identical: human beings, for instance. There is nothing in the essence of a human being that requires that it exist in fact.
But, we notice, this is true for everything material, since there is nothing in the material universe in which existence and essence are identical. Why not? If this were true, something in the material universe would have to exist necessarily, i.e. it would be impossible for it not to exist, simply by what it is. But I could not exist, since I depend on my parents for my existence. Similarly this bookshelf to my right could not exist, since it depends on the actions of the people who made it for its existence.
These causal series are known as per accidens - a father has a son, who has a son, who has a son, etc. In them, the causal power or ability to continue the series exists independently in each entity of the series. Thus, a father can have a son and then die, but the son can still continue the chain by having a son of his own. The son does not depend on the father to have his own causal power, except in the sense that he depends on the father for his existence.
But there is another type of causal series, which is ordered per se. In this type of causal series, everything functions instrumentally as a “proxy” of whatever really possesses the causal power. So, for instance, my hand moves a staff which moves a rock which moves a leaf. This is a per se causal series since the leaf could not move without the rock moving it; the rock could not move without the staff moving it; the staff could not move without the hand moving it. Each entity in the series depends on the previous one for its causal power.
But in a series like this, we must have some sort of first entity which has the causal power and gives it to the entities in the series. Consider, for example, a paintbrush, and divide it into segments. The bristles are moved by the segment closest to them, which is held up and moved by the next segment, which is held up and moved by the next segment, etc. but all of these must be held up and moved by the hand, which is the source of causal power for the entire series. So, to claim that a per se causal series can simply consist of infinitely many elements which depend on the previous one for their causal power would be, in this concrete setting, saying that the brush can paint by itself if it has an infinitely long handle.
This is clearly not the case; there must be a first entity in the sequence. Now let us return to the analysis of essence and existence. In many things we see that essence and existence are not identical, which means that they depend on something else for their existence. If a bookcase, for example, is sustained in existence by the wood molecules which make it up, and those are sustained in existence by the bonds between atoms, etc. then we have a case where something in which essence and existence are not identical (the bookcase) are sustained by something in which essence and existence are not identical (the molecules) which are sustained by something in which essence and existence are not identical (the atoms), then we can see that this is going to be a series in which everything depends on the previous entity for its existence. This must arrive at a first member of the series in which essence and existence are not distinct (since that requires that something else is the reason for or source of its existence), but in which essence and existence are identical, since that means that what it is is identical with the fact that it is. Thus we must have something which could not not be, or which necessarily exists, and from which all other things acquire their existence.
Note that I am not arguing that this being is only the initial source of the universe’s beginning, although I would argue that. I am saying that this being currently sustains everything in existence right now, since in per se causal series the relationships are simultaneous. It is the source of all being and it continues to be the source of all being. Of course, we call this being in which essence and existence are identical God, and one can continue to make the arguments that God is personal, one, etc. but that would take more time.
(But first you have to know the subject matter well enough to point out the logical inconsistencies presented by the person you are refuting.)