The Bible does not say the earth is flat. Ask him for chapter and verse. "The Lord sits above the circle of the earth." That can mean globe or circle. "Firmament" is what we call "atmosphere". It separates the waters above (clouds) from the waters below (seas). "And God made the firmament, and divided the waters which were under the firmament from the waters which were above the firmament: and it was so."

The fact that the "firmament" (or "expanse") is also described as dome shaped is only consistent with a spherical earth. Otherwise, the clouds would be lower near the edge of the disk. Flat earthers deny that there is actually a base in Antartica or any Antartic at all - a govt fiction. The sky meets the earth at the ice wall in their view. There are tours around the edge of Antartica for about $10K - but not likely to convince them. Visits to the base are not generally granted to flat earthers - too much of a liability. Balloon enthusiasts report that you can see the curve from 30000ft (oxygen and extreme cold protection required), which you can't from the narrow view of an airplane window. That is about $900. They would probably interpret what they see as a disk.

The main proofs are metrical - shortest air routes become more and more circular the farther south you go. (Common polar routes look fairly straight on their artic centered map.) The problem is, they can't understand that kind of geometry. The key question is: if you write a sci-fi novel where the heroes are transported to a spherical world, what measurements do they make that makes them realize this astounding fact? But flat-earthers are too innumerate to answer this.