To begin, I want to commend you in your persistent prayer for your friend. Your acknowledgement that it is God who acts and changes hearts / minds is vital; starting from your knees utterly changes the possibilities. You are off a good start, brother.
You have had some good insights shared here already: turning the claim in on itself can logically compelling, as @Mercury suggests, and @blbossard is correct that demonstrating the experiential shortcoming of the proposition can expose the weakness of this kind of assertion about reality.
For good measure, I’ll throw in another option. It is a bit off the beaten path from the classic and clean apologetics logic moves, but instead of trying to play by rationalist rules, I sometimes prefer to get in the weeds about the definitions of reality and what counts as real, as knowledge, as fact. I think stepping back and trying to define terms is always critical to fruitful conversation (Socrates made a full-time job of it) and can be a very revealing conversation in itself.
I have shared a bit about this foundational level of questioning in a conversation thread here. It is about getting at the issue of epistemology (how we know) to reveal assumptions and open up new possibilities.
I grant that its not as elegantly slick as pointing out a self-defeating position or exposing someone’s lived /practical denial of a proposition they have verbally/theoretically asserted! But in philosophy I am more continental and existential than analytical, given more to prose and dialogue than syllogisms! This is what I have, so this is what I share:)
So often, a good question is like a burr under a saddle…whatever you argue can go like water off a duck’s back, but a question lodges in the mind—the heart—and keeps interrogating, persuading…
I wonder if—as is often the case—a really thoughtful question would help your friend see his own assumptions and open him to new explorations with you? What do you think? After prayer and thought, I wonder if any questions come to mind for you?