They are using text inputs, where you press enter to send stuff daily. Most of the hurdle is just overcoming the preconception that at black in put window means hard mode.

They can right-click in the folder view of their OS file viewer. On Windows they can also just type the command into the path bar.

When you tell them the command, you could also just install it. Also you could just tell them to type the name of the app 'ffmpeg' into the OS store and press install. They do this on their phone all the time.