Fisheye lenses produce a distortion that makes straight lines appear to curve away from the center. Your friend's argument is perhaps that pictures of a curved horizon are just demonstrating this artifact. Indeed, many hobbyist rockets flown with a GoPro on board show this behavior while not reaching enough altitude for curvature to be reasonably visible.
When horizon curvature is due to a lens effect, then when the camera pans such that the horizon crosses the center of the image, we expect the horizon's curvature to switch as well. But there are plenty of photos and videos taken from enough altitude that demonstrate that this doesn't occur. So at least these are not showing curvature just due to use of a fisheye lens.
Now perhaps those cameras were purpose-made so that they bend everything the same direction. Or maybe the pictures are photoshopped! Or maybe you're just looking at the screen wrong! You can come up with an infinite number of reasons not to believe in picture evidence. In fact, you can come up with an infinite number of reasons not to believe any evidence at all. It's just that when you go to do any activity that actually requires taking into account the curvature of the earth, you will be wrong.
Many of these activities are hard to do, for instance: building a GPS receiver, navigating a ship across an ocean, or sending a spacecraft to the moon. But there are easier things that immediately require spherical trigonometry, such as: predicting time of sunrise and sunset for any position on earth and any time of year, or predicting which stars will be visible in the sky, or where the moon will be. To carry out these calculations and still believe the earth is anything but near-spherical requires willful misimagination.
Perhaps a better question is: suppose the images were taken with a fisheye lens. So the horizon is actually straight and not curved. But if the earth were hollow why would there be a horizon at all? In images from space, when you look at the horizon, you see the ground stop, then the sky begin, then space begin. You wouldn't see black there if the earth were hollow. So this fisheye lens claim actually rules that out. If your friend really believes all these photos are real but were taken through a fisheye lens, perhaps he should at least be arguing that the earth is flat.
I wrote all that out in case you really wanted to read a response to this fisheye lens claim. Most likely none of that will be useful for your friend. All the other commenters here are basically right: it's less likely that your friend is wrong out of ignorance (correctable with more information) than that he is pranking you or will never stop being wrong.
penlu t1_j9rgygt wrote
Reply to comment by YeetFleekMasterOfRap in Help! My friend is convinced that the Earth is hollow and we are living inside of it! by YeetFleekMasterOfRap
Fisheye lenses produce a distortion that makes straight lines appear to curve away from the center. Your friend's argument is perhaps that pictures of a curved horizon are just demonstrating this artifact. Indeed, many hobbyist rockets flown with a GoPro on board show this behavior while not reaching enough altitude for curvature to be reasonably visible.
When horizon curvature is due to a lens effect, then when the camera pans such that the horizon crosses the center of the image, we expect the horizon's curvature to switch as well. But there are plenty of photos and videos taken from enough altitude that demonstrate that this doesn't occur. So at least these are not showing curvature just due to use of a fisheye lens.
Now perhaps those cameras were purpose-made so that they bend everything the same direction. Or maybe the pictures are photoshopped! Or maybe you're just looking at the screen wrong! You can come up with an infinite number of reasons not to believe in picture evidence. In fact, you can come up with an infinite number of reasons not to believe any evidence at all. It's just that when you go to do any activity that actually requires taking into account the curvature of the earth, you will be wrong.
Many of these activities are hard to do, for instance: building a GPS receiver, navigating a ship across an ocean, or sending a spacecraft to the moon. But there are easier things that immediately require spherical trigonometry, such as: predicting time of sunrise and sunset for any position on earth and any time of year, or predicting which stars will be visible in the sky, or where the moon will be. To carry out these calculations and still believe the earth is anything but near-spherical requires willful misimagination.
Perhaps a better question is: suppose the images were taken with a fisheye lens. So the horizon is actually straight and not curved. But if the earth were hollow why would there be a horizon at all? In images from space, when you look at the horizon, you see the ground stop, then the sky begin, then space begin. You wouldn't see black there if the earth were hollow. So this fisheye lens claim actually rules that out. If your friend really believes all these photos are real but were taken through a fisheye lens, perhaps he should at least be arguing that the earth is flat.
I wrote all that out in case you really wanted to read a response to this fisheye lens claim. Most likely none of that will be useful for your friend. All the other commenters here are basically right: it's less likely that your friend is wrong out of ignorance (correctable with more information) than that he is pranking you or will never stop being wrong.