The flat earther could just say the curvature is a result of the curved glass window in the airplane. Or something. Installed on purpose to fool us. I mean, it's not like these people will suddenly get conspiracy-shy just because you put them in an airplane. :D
I liked the video, and if I understood the overall argument correctly, it might be similar to that Wittgenstein anecdote:
“Tell me," Wittgenstein's asked a friend, "why do people always say, it was natural for man to assume that the sun went round the earth rather than that the earth was rotating?" His friend replied, "Well, obviously because it just looks as though the Sun is going round the Earth." Wittgenstein replied, "Well, what would it have looked like if it had looked as though the Earth was rotating?”
In other words, you're saying to the flat earther: if the earth was a ball, would it look any different to what you're experiencing right now? Take a zoom lens and look at a baseball real close, or else look at Google Earth and don't take it to be the Earth, just take it to be some ball, and see how your experiences are replicated on it...
Yep that's a good quote! You could maybe get a basketball and once heavily zoomed onto a snooker ball, rotate it ar... wait this experiment would be incredibly hard to set up the more I think about it.