OP made a single, simple, clean, clear argument based on an easily replicable experiment and simple high school trigonometry - and you respond with an a priori dismissal of the whole theory in the form of a link to a "wiki" known to be extremely biased and dismissive of anything that does not regurgitate "conventional wisdom" which does not address OPs argument in any way.
You put no effort into thinking or making a sound counter-argument. That is not how to combat misinformation.
I am looking forward to your next try :)
Is there any source they wouldn't find "extremely biased?" No, because that's how crackpots work.
Seriously the only way I could actually disprove this, is to build a spaceship, take him into space and fly him around the entire planet so he can see the shape. Then he'd probably claim I drugged him and used hypnosis to plant the memory in his brain instead or something, and that I'm part of the vast conspiracy.
Meanwhile airlines are throwing away millions of dollars by following curved flight plans on a "flat earth." Because we all know corporations love conspiracies more than money!