Put your kit lens at 35mm for a few days and don't move it when you shoot. Then put your kit lens on 50mm and shoot for a few days. You'll realize which one you'd use more. I have a 35mm and use it far more than my 50mm but it all depends on what you take photos of the most. I like both lenses its just for me the 35mm gets used far more. Hope that helps a bit
A possible better way might be to analyze the EXIF data in pictures that she's already taken, to see what focal lengths she's been using. I'm assuming that the D5100, like my D3200, saves, in the EXIF data of each picture that it takes, the focal length to which the lens is set that took it.
As an experiment, on my Macintosh, I typed the following line:
exiftool ~/Pictures/-D3200/2013/* | grep Focal\ Length | grep -v equiv | grep -v In\ 35mm\ Format | grep -v Min | grep -v Max >/tmp/__untitled/Focal.txt
Note that this depends on having
exiftool installed, which may be downloaded from
ExifTool by Phil Harvey, and it depends on
grep, which is native on a Macintosh, as on any Unix-based system, but which you may need to find, download, and install, if you're using a Windows-based system; you're on your own as far as finding and installing a Windows-based version of grep; as well as translating the Unix syntax of the above line into a comparable Windows syntax.
Importing the resulting file /tmp/__untitled/Focal.txt into Excel, I was able to produce the attached spreadsheet, from which I was able to determine that of the 99 pictures in that directory, I uses my kit lens in the 38-55mm range more than three times as often as I use it in the 20-38mm range, and therefore, if I was contemplating the purchase of either a 35mm or a 50mm prime lens, I'd probably do better to go with the 50mm.
Interestingly, of the ranges in which I divided up this analysis, the second-largest group is “0.0 mm”, meaning that I didn't use the modern kit lens at all, but one of my stone-aged non-AI lenses for my equally-ancient F2; being as primitive as they are, they do not report their focal length, so it gets recorded as zero.
View attachment Focal.zip