What would it take for a Flat Earther to be convinced that he is indeed wrong?
As a second idea, do you believe that the land we stand on goes on infinitely?
best method: dying. either there will be nothing or something completely different he believes or maybe his religion was correct, which i doubt, but still he will find out that his gold will tell him: sorry dude, i thought a spherical earth was just prettier.
until then, many are lost in dogmatic belief.