I think he already owns a D800/600.
I'd wait and rent an XT1 once the firmware update is released.
One thing that will be different is having only the APS-C / DX crop sensor which changes your lens selections.
For me, I shoot mostly using telephoto lenses and rarely need extra high quality wide shots so having the extra 1.5 crop is a bonus.
A good friend of mine has the D810 and raves about it's low light capabilities and excellent high ISO results.
We did a cursory sharpness comparison test between my D300s, X-T1, and his D7000 & D810 using their 18-55 kit lenses. It appeared the Nikon 18-55 had superior sharpness albeit marginally.
We didn't have time to try it with the same lens Nikkor lens on both cameras using my Nikon to Fuji lens adapter though.
HTH.