Get rid of the fireplace. Or put the TV in front of the fireplace, building a facade to cover the fireplace and complement the TV. If they want to spend money, install a motorized mount that will lift the TV from behind a picture or mirror above the fireplace and then drop it down in front of the fireplace.

Otherwise, TV above the fireplace is just stupid. Uncomfortable and looks super super tacky. Urge them strongly not to do it unless it's for background noise or use only during parties where everyone will be standing. And in that case it really should be hidden from view when it's not on and outside of those occasions.

People that do this are those who don't know any better (so you should teach them) or watch design shows that no one should be watching. As in a designer that puts a TV above a fireplace shouldn't have a job. You'll probably notice that all these TVs will also be showing the incorrect aspect ratio.