Between the two quoted,
The first decision will be the lenses that she will want to buy in the future, depending on the type of photography that she is interested in - good lenses, glass, are expensive and should take a bigger part of the overall £ budget than the body
Then the size of the Nikon Sensor, Full Frame, (FF), which is approximately the size of the old film 35mm negative
or a cropped sensor, known as DX in Nikon terms - the sensor is smaller than FF
Specific lenses for the DX sensor are very good but cheaper than lenses that can be used on a FF Body
The D500 is DX and the D780, which replaced the D750 is FF, (Full Frame)
FF bodies are more expensive than DX Bodies
"She enjoys landscape photography and I understand the D780 is full landscape" - the size of the "Landscape" is determined by the "size" of the lens, (angle of view, if you like), plus of course your distance from the subject
35mm on an FF body is 35mm but on a DX body in angle of view terms it is 35mm x 1.5 = 52mm "ish" - so with the same lens a FF Body would be better for Landscape, i.e. give a wider angle of view
all the above comments are in "general" terms