There’s a reason your Biblical flat earth friend has tried to tell you about the Firmament…
This vaulted dome ceiling that encapsulates the circle of the earth was the only thing God created on Day 2, in the first chapter, on the first page of the Bible. He gave this Firmament the name “Heaven” in Genesis 1:8, while describing the setting for the story of mankind.
The Psalmist says the Firmament shows us God’s handiwork. In this way, it is a vital component in recognizing the truth of God’s creation. Its existence is overwhelming evidence for intelligent design – which is why Atheists are testifying, in droves, that Biblical Cosmology is what God used to lead them to faith in Christ.
And given that the definition of a basic dome shape requires that it has a flat, circular base… the Firmament is a blatant giveaway for the shape of God’s green earth.
Heaven is a literal and physical, solid structure above a stationary, level plane. Your friend just wants you to see the glory of God’s work.