Bloody hotdogs!:
Great question! I agree with everyone on this page the argument he present will be special pleading, however you should be aware there are ways to postpone the special pleading which he may pursue.
If you are interested in this type of stuff, you should really check out the christian philosopher William Lane Craig who has done much to popularize this type of argument. The way he would answer the argument would properly be: "Because God is eternal, so it does not make sense to ask where he came from anymore than it makes sense to ask for a square circle".
Now, this does not really answer the underlying point your question raised: "If god can exist eternally why can't the universe (or some other reality)?" and this is in my opinion too a fatal flaw.For this reason the argument is typically presented so as to first establish the universe *is not* eternal but must have had a prior cause -- which can then be god. Notice this shifts the discussion to discussing cosmology and specifically what *cannot* be the case in cosmology rather than presenting a positive argument that god is eternal and created the universe. This is a nearly universal tactic in modern apologetic and I imagine your elder is going to go down that route.
The arguments that the universe is *in fact* not eternal are based on bad physics (a miss-application of the Borde Guth Villenkin theorem or the second law of thermodynamics), bad math (miss-application of transfinite mathematics) or quote-mining (the big bang is often said to be the beginning of the universe so this proves the universe began to exist). Notice these are rhetorically *very* effective arguments and you should be well-prepared to what he might say.
The best way to counter these arguments is that whenever he present and argument why the universe cannot be eternal you must turn it around and ask yourself (and eventually him) how *theism* solves this problem. For instance, he might say the universe cannot be eternal because it would then have to contain an eternal chain of cause and effect and such an actual infinite chain cannot exist because it would be absurd (his reason to think this is absurd does not matter at this point). However if this is absurd, supposedly god has existed for infinitely long time, and thought an infinite number of thoughts, so why does this argument not just rule out God?
At this point even William Lane Craig resorts to special pleading: Because time works different for god than for our universe.
