I assume you are talking about common descent, in that case it is a historical hypothesis, not a theory.
To be precise, we are talking about the historical hypothesis that evolutionary mechanism are capable of bringing forth multicellular life, 100% of the diversity of species, complex biomechanical organs and biological systems, consciousness, and intelligence from the ancestors of a hypothetical proto-cell randomly through gradual steps.
There is no biology being done which requires evolution as a premise in its hypothesis, and without which premise the science could not proceed. To claim that it is fundamental and imply that it is necessary to biological science is a genetic fallacy. Biologists by and large try to understand existing systems and structures; what they’re made of, how they’re constructed, and how they function; how they evolved or history of the fossil record 200000 years ago, or extrapolatory hypotheses about entities which do not exist in the current time frame is beside the point. Stem Cell biology is booming due to observation, hypothesis, and experimentation, not by attachment to evolution. As you are trying to make him believe this fundamental 'truth', you want to shame him into thinking that he is anti-science and therefore irrational if he doesn't accept evolution, you are trying to ignore the existence of flaws that are required in order to fill out the entire worldview, including the inability to answer the question of what life actually is, what the source of first life was, why life perpetuates itself despite being subject to entropy, what is the source of mind, rationality, agency, and self awareness, along with many other questions. It is patently false that it is not fundamental at all when it can't answer the fundamental issues.