She doesn’t install apps, Her config is what she needs. But nixOS install is pretty simple if you can copy paste text.
You go here https://search.nixos.org/packages
Search for a package, and click if you want permanent or ephemeral app and paste the code into the shell or into your config file.
Run a rebuild
Pretty easy
Yeah, I find it simple, but I’m used to text edits driving batch files etc. Daily driver I use Tumbleweed, the Yast zypper GUI gives you select and apply for pqckages, no command line needed.