Truthfully, I thought the 50mm was related to the width or aspect ratio, not the magnification. I had gotten that recommendation from our wedding photographer who I became friends with after. He said the 50mm 1.8 and the Tamrom 18-270 were his two favorite lenses. Stupid me didn't ask him to explain why or what that meant, I just started writing down numbers and plugging them into google... I picked the 50mm 1.8 because I thought it would be a more simple lens to start with since it wasn't a zoom lens (and probably subconsciously because I'm a cheap-skate and it was the cheaper of the two). Thanks for clearing that up. I don't know about dropping $4-500 on the 18-270 right off the bat, I'll look more into the 18-105 for starters and let her upgrade as she needs it....
If you also had a zoom lens, then you would be covered, and I withdraw all objections to also having a 50mm, as you see fit. The zoom will also do 50mm, but the downside of the zoom is that it is not a f/1.8 lens. f/1.8 might be useful a time or two in its life, but the 50mm lens cannot do 18mm wide angle nor telephoto. Which you will need much more.
Expensive zoom lens might be f/2.8, but less expensive ones might be f/4, and worse, when zoomed longer, cheap ones might be only f/5.6. That is not great either, not in dim light, not with flash, but it is little issue in the sun. In shade though, it shows up, and lengthens shutter speeds. So a f/5.6 zoom is not great either.
The real advantage of the f/1.8 or f/2.8 lens is that they should perform very well at a useful f/4 (like with flash) where the f/4 lens would be wide open then (not a good thing). And a f/5.6 lens cannot even do f/4 (but most flash pictures will not be telephoto).
But the main point was, it would be extremely limiting to have only a 50 mm lens... esp on DX, where it acts like a 75mm telephoto on FX or 35 mm film.
Get a zoom lens first, and then contemplate the 50mm in the spring or whenever. Have a reason to need it first.
Go back to the store, and play with the zoom lens a bit. Zoom to one end, and the other end, and to points in between, and see the view it offers in the viewfinder (which will be the same view your picture shows of course). Understand the versatility it offers, in one lens. My opinion is she cannot do without it.
This may be a complication to mention at this point, but FWIW, you can also see this zoom effect on any picture in your photo editor. As you "zoom in" on that picture, it increases magnification, which necessarily crops to reduce the width it can show in the same borders when enlarged. Same thing as zooming focal length magnifies the view in your camera frame borders. Except cropping afterwards discards pixels, where zooming in the camera keeps more pixels (which is more detail).
As to the picture you take... speaking of of the width of the field of view:
An 18 mm lens on DX will capture a horizontal view about 66 degrees wide (roughly 2/3 of a 90 degree angle)
30mm lens on DX about 45 degrees wide (half of a 90 degree angle)
50mm on DX about 26 degrees (roughly 1/3 of a 90 degree angle)
200mm lens on DX about 7 degrees wide.
Doubling focal length is 2x magnification which cuts the picture width in half.
So, you adjust the zoom lens to show the picture view that you want to capture. It is an extremely strong tool to get the picture you want. Or, less convenient, but you could instead swap the lens to another one of a different focal length, if you had one. If your only lens is a 50mm, then you don't.
One more caution... just to complicate things.
If some hypothetical zoom has say a 20-60mm range, that is called a 3x zoom (60/20).
If it has a 20-120mm range, then 6x.
If it has a 20-300mm range, then 15x.
These large x values seriously complicate good lens design, having to do all things at once. This is one advantage of the prime lens, say 50mm (prime design only worries with that one focal length). But it can only do the one.
When the zoom lens design passes about 6x, then image quality and sharpness suffers a bit. So this again is a trade off, convenience of 10x or 15x in one lens, or the most perfect precise results possible in a lens of less range.
For example, the 18-200 zoom was not Nikons best lens. This extreme 11x zoom design "loss" is less important if you only show it small, resampled screen images or print only 4x6 inches.
But if you view it large (cropped or printed), then we need all the sharpness we can get.