I second talking with her, and having her feel the cameras.
The D850 is significantly heavier than the D7200. And depending on what, where and for how long she is shooting, that weight increase can be a negative factor. Example if she is carrying her kit for miles, when shooting wildlife, I would think that weight would be a factor for her. It is for me. Everything is compromised to minimize weight and bulk.
To get the same image magnification out of a FF/FX camera, she would need a longer (and heavier) lens than a DX camera.
On the DX D7200, a 70-300 has about 8.6x magnification at 300mm.
A FX camera, would need a 400mm lens to get to the same 8x magnification. So the big Nikon 200-500.
Keep in mind that, holding the camera+lens for 5 minutes in the camera shop, is very different than carrying and holding the camera+lens for a few hours in the field.
One of the reasons I shoot a 70-200/4 rather than the f/2.8 lens, is because the f/4 lens is HALF the weight of the f/2.8 lens. And when shooting 2 sequential football games, at the end of the night, that weight difference makes a difference in how tired/sore I am.
So if she wants FF/FX, also look at the lighter D750. The D750 is still an FX camera, but lighter than the D850.
Rather than the D7200, I would look at the DX D7500. Here are my 2 primary reasons:
- The D7500 can use the new AF-P lenses, where the D7200 had limited functionality with the AF-P lenses.
For me the biggest issue is, with my D7200, I cannot turn VR off, so I cannot use the AF-P lenses on a tripod. And for me, that is NOT an acceptable restriction.
- The D7500 has a pivoting rear screen. This gives the ability to shoot high angle shots without a ladder, and low angle shots without laying in the dirt/mud.
To me the D500 is more of a sports/action camera.