The fixed_aspect_ratio render script that you are basing yours on mentions this in the readme:
“The area visible using the original screen size will always be visible, either centred if the window dimensions are larger, or zoomed out if either window width or height is smaller than the original size.”
This means that on your friends high resolution screen you’ll see the area defined in game.project and everything around it. It will not zoom in. You could modify the render script so that it zooms in if the screen resolution is higher than defined in game.project and zoom out if the screen resolution is smaller (this it already does).
If it’s a 4k screen it really has to have a pretty beefy GPU to be able to manage 60fps. Could you post a screen shot of how your game looks? Have you tried running the on-screen profiler, or even better, the web profiler against the phone?