FE theory is proven wrong in every aspect. In case of sunset, I always say them this observation:

According to their theory the Sun is always above FE. In moments of sunset, sun is just moving away from your point on Earth. In one moment it just went too far and you can see it anymore. That is their theory of sunset.

If that was the case, in moment it went too far, you could just take binoculars or telescope and look at direction of sun and you should be able to see it. Of course you can't see the sun after sunset because Earth is not flat. You can't see the whole sun in one moment and in next one don't see it at all. But of course their belief is stubborn so they ignore all evidence like this because they point in opposite conclusion.