I would say he can. He just needs to open a web page with a lot of text. The difference in sharpness will be noticeable immediately for somebody that is used to more than 326ppi. Also the difference in contrast/blacks is still quite obvious. And I doubt the human eye can perceive "color accuracy"(whatever that means) on these screens. Anyway iphone X's screen is classified as "indistinguishable from perfect" in terms of sRGB compliance so i don't see what advantage in terms of color could the XR's display have.