I second the plane idea. Though actually the seat doesn't matter since he'll want to look out both windows to see the drop in both ways. Else he can just say, "The plane was tilted."

The connection between height and distance that this shows is why old ships used to have a crows nest. I love that connection, and you know how to do the calculation for why it matters. But proving that it is right is harder, particularly if you don't live near a large body of water.

With a bit more trig and a map you should be able to figure out the angle above the horizontal that a mountain somewhere in the visible distance should be. That angle should not be the same as it would be if it was flat. Verify it. Go driving somewhere else, verify it again using the same mountain.

But honestly, I'm personally most convinced by the fact that a phone call to someone in a different time zone makes it easy to verify that you're pointing different directions relative to the Sun.

As much as people like to point to things like eclipses, that relies on knowledge that you cannot verify yourself. I prefer verifications that you can duplicate with direct observation, without relying on outside experts. Because flat Earthers do not trust experts.