Since you’re a science enjoyer you should start off with the scientific method. What is a consequence of living inside a hollow earth that one could test that isn’t better explained by another model such as those currently used? Seriously, think this through. Be aware though that there are subtleties that crop up in every investigation that will complicate finding the correct answers. Even Einstein had to deal with this, he continually published the answers he found to questions that seemed to disprove his claims until nobody could think of any that couldn’t be answered. That process is still going on today, scientists don’t predict the conjunctions of Jupiter’s moons or solar eclipses or the temperature of craters on the moon to prove how smart they are, they are continually observing to see if the real world disagrees with the predictions made by their models. If it ever did and they could show it with evidence that would be a history making event and any scientist would be thrilled to be the one to make such a discovery. That’s why the whole idea of science having a “narrative” is so wrong headed. If your friend can demonstrate the hollow earth better explains what we see when we look out at the universe than Copernicus, Galileo, Newton, and Einstein, science will agree the earth is hollow and we live inside it. Personally my money is on the other guys though.