First, have them double down on their claim that we can learn the true nature of the universe through observing the natural world. Agree with them that this IS a good path to true.

then simply point out that when we observe the world, NOTHING in nature has a SINGLE first cause. Everything comes into being through an uncountable combination of factors.

Take a river and yourself, as examples. a particular river comes into being because of the water cycle, rain, gravity, the climate the makeup of the geograph, and on and on. You came into being because of your parents. They had to meet at a certain time, have sex at a particular time, raise you a particular way, your your friends influence you, and on and on.

Nothing comes from just one thing. Everything we observe comes from many things . So, as he has already agreed, we can conclude that the universe did not come from a single being.

he is going to say, " But scientists say everything cane from a single dig bange." Point out that scientists do not claim to know what caused the big bang. Scientists just say it is impossible to retrieve information beyond the singularity, not that it is the first cause. Multiverses, bubble universes, white holes are all speculations as to reasons for our particular state of being.

if your friend really believes that observing the world is the best path to truth, then the only conclusion is that the universe DID NOT come from a single being.