I agree with redsnappa’s sentiment, although to answer your question the D850 has a really nice screen, very detailed. As for the graininess of the screen, all LCD screens whether the one on the back, or the ones in the viewfinders on mirrorless cameras will get grainy in low light. Some are better than others, but I can’t say I’ve noticed the D850 being any worse than other cameras I’ve used.
There are other factors that will affect how grainy the screen is compared to one you’re testing side by side, and that’s the lens aperture and whether you have exp preview on. If for example you have exp preview on and stop down to say f11, and your mate has only stopped down to f4 you will have 8 times less light entering the camera (assuming you both have exp preview on) which will make it far grainier.
BTW there is a D850 thread already and if you have any questions in the future you might get more responses if you post in there