[munk-a]

I am assuming (probably falsely) that if you assume the earth is flat then you assume there is an edge somewhere - not anywhere but in some particular spot. If the assumption is that it's around the south pole (as per the map in the linked page) then a flight from Chile to Australia would be among the longest flights possible, it would be hard to argue[1] against a round earth when that flight took a few hours when Bejing -> Melbourne or even London -> Melbourne is a clearly shorter according to their flat earth map.

As per my general ambivalence to disproving their ignorance though - at this point they might suspect... anything really, maybe they were secretly drugged during the flight to feel like it was really quick and their watches were adjusted or maybe the lizard people used their warp technology to teleport them part of the way. Honestly, willful ignorance gonna willfully ignorant - a conspiracy theorist that's in deep can always find a new excuse.

1. Argue logically that is... willfully ignorant people gonna willfully ignorant as mentioned in the comment above