I see that JRiver can convert audio or video but not image files.
This is incorrect. MC can convert to jpg or png.
If you use the 'Replace original' option, MC will preserve any of its own associated library data, with the exception of the Date data, which will be overwritten.
If you use a different location to write the converted files to, zero metadata will be preserved and the converted files are not imported into the MC library.
These are probably not ideal outcomes, but, poor data preservation aside, MC
can convert the nef files
The family friend who chooses to shoot in RAW format is surely the best place to go? If they're shooting in RAW, it seems fair to assume they're doing their own post processing and as such, should be able to provide you with exported jpgs that will have all associated EXIF and IPTC data embedded. Though, they may have no understanding of what the RAW format represents, else they surely would have done this in the first place?
Beyond that, wer's advice is sound. A favourite of mine outside of Lightroom is XnViewMP (
https://www.xnview.com/en/xnviewmp/#downloads) which is free for personal use, and includes the very handy ExifTool. It can convert nefs to jpg, producing files with any included exif data.
-marko