There are ereaders with screens with 600x800 pixels and tablets with 768x1024 pixels (yes, there are some other formats too). The aspect ratio of the screen in both kind of devices is 1,333, so the cover should have this ratio too.
When resizing a picture there is less loss of quality when downscaling than when upscaling. So in any case you should choose the largest version of your picture. And, here we are again with Amazon: For KDP they want a cover with a height of at least 1000 pixels.
Which means: The width of 800 px (your largest picture) needs a height of 800x1,33=1066 px (1131 px is too much). But there could be a menu bar and a status line, as DaleDe mentioned, which means 1066 px minus about 66 px. So your designer friend has to cut off some pixels at the top and/or at the bottom of the picture with a resulting cover of 800x1000px. On an ereader it will be shown with 600 x 750 px only, but the cover will be shown in good quality also on a tablet with a larger screen.
I'm afraid there will be bars in any way depending on devices, because you don't know if they have menu bars or not. But if you get a blank bar at the bottom, it will be so small that it will be acceptable and you still have the original quality of the picture.
Resizing the cover by a device to full screen with a different aspect ratio of width/height will cause distortions. Maybe this is not apparent in the picture, but you will see it in the letters. I would prefer the first version with width=100%.