Fake Moon Landing
One way to introduce people to the flat earth is by talking about the moon landing. You don’t start off by saying it was fake but you introduce facts that makes you friend start to question the narrative. For example, ask how Gus Grissom was able to talk on the microphone with a background noise of 140 decipals. Then introduce another fact. Of course, you can just show him this or other videos.
We know the moon landing was fake because there is no other planet out there. I had learned about the fake moon landing years before I heard about the flat earth and I’m sure you did, too, if you were investigating government lies at an earlier date. Then, when I learned about the flat earth, it makes sense that the moon landing was fake, too. Unfortunately, there are good people who are Truth Seekers that know the moon landing was fake but still believes in the heliocentric model of the universe. God willing, in time they will be open to know about the flat earth.