Easiest way to prove it? The phases of the moon.
How is it proof? As the moon rotates around the planet the Earth's shadow controls what phases of the moon we see. The shadows on the moon that make it a crescent moon from here is visual proof of the Earth's curve. The moon's orbit is not constant. The orbit is constantly changing, but the phases are not. If the Earth was flat, the phases of the moon would change every 8 years along with it's orbit, and the shadow put on the moon would produce some strange Pac Man shaped phases or have full moon for a month or longer depending upon when the Earth is around the sun.
Super simple.
Steph is a basketball player and I'm currently thinking he's an idiot outside of that arena. Considering all of the things we're capable of doing with our current technological capability that he thinks traveling to the moon isn't possible is a bit dumb.