I imagine you could best respond by asking the person the reasons for holding to that position. The Bible does not support that view.
It could be that people imagine the world to be flat from reading passages like Isaiah 11:12 (“He will raise a banner for the nations and gather the exiles of Israel; he will assemble the scattered people of Judah from the four quarters of the earth.) and Revelation 7:1 (After this I saw four angels standing at the four corners of the earth, holding back the four winds of the earth to prevent any wind from blowing on the land or on the sea or on any tree.).
In his word, however, God often uses language to come down to our level so that we can understand him and his ways better. The “four quarters” and “four winds” match up with the four directions (Luke 13:29). In the Bible God even used the terminology of people’s understanding of his creation (cf. Joshua and the sun standing still, Joshua 10:1-15).
You may find helpful resources from Lutheran Science Institute to formulate your answer.