What's the cheapest lens to buy for showing ISS to a flat earther? Recently I entered a discussion with someone that has a relatively popular YouTube channel which says among other things that the earth is flat. I said that earth isn't flat because you can see the ISS by yourself through the telescope. I was reminded that I'd only seen it on the internet, but not actually seen it directly myself. I was told that if I buy a telescope and film by myself they will also buy one, do the experiment, and post the footage on their YouTube channel.

So, what's the cheapest telescope that I can buy and record with a cellphone? I've seen people using a Nikon P9000 but what matters is its lens, which is already expensive, and the camera is expensive too. Maybe it's possible to buy a cheap photography lens that has both good quality and high zoom and can be used with a cellphone?
I think you'd want a focal length of 700-1000 mm, and the cheapest way to get that would be a basic Newtonian reflector of 100 mm aperture.
Examples: Meade Polaris 114mm, Celestron PowerSeeker 114EQ, Orion SkyQuest XT4.5.
There are brackets available to mount a smartphone on any telescope.

Before trying to track ISS, practice acquiring easy targets such as the Moon, the Pleiades, etc.
Then use Heavens Above to predict ISS passes visible from your location.
After you collect your evidence and your hyper-skeptical acquaintance rejects it, you can still use the scope to enjoy the planets and a few dozen star clusters, nebulae, and galaxies.

On the other hand, maybe you don't need a telescope.
ISS is visible to the naked eye, is silent, and doesn't appear to slow down as much near the horizon as aircraft do.
Seeing it disappear as it enters Earth's shadow could be persuasive.