See the picture above, show this to your globe earth friend and ask this question: “Why is there is cure on the horizon?”
If he say it’s because the earth is a globe, then ask him, “If that is so, why don’t you see it on all pictures of this altitude?”
He might start scratching his head. While he is doing this, say, “I’ve seen pictures of the horizon taken 35,000 feet high and the horizon is flat. Why isn’t it curved there?” Wait for a response.
Going back to the photo above, you can add, “If this is really the curvature of the earth, it looks like we have about 10 degrees of a circle, in mile. That would mean that the whole town would just fit on the whole earth with no space for anything else. In short, the curve is too great.”
He might say, “Well, they actually used a fish-eye lens or something like that.”
You can add, “Why did they do that when the photographer could have used a normal lens that showed how it really looks from the air?”
He might get flustered at this point and say, “This is to show you that the earth is curved.”
You can finally add, “If the earth is really curved, it would show up with a normal lens. By the way, do you really think the earth is a globe?” Then wait for his response. Anyhow, I hope that a conversation would help your globe believing friend – provided his I.Q. is higher than 50.