We don't support 3rd party images, especially the ones loaded with ROMs - as are the ones that you're trying to choose from. Use the image from retropie.org.uk/download and follow the installation instructions from the Docs.
If you're hard pressed on time, I recommend you get your brother friend a ready made console, like one of the mini consoles released by Sega/Sony/Nintendo, which come already installed and with the games included.
I'm not sure the download location has changed, I don't see the doc page reference a precise location for each system. The cheats are downloaded from the Libretro buildbot AFAIK, not directly via git, the archive is generated from https://github.com/libretro/libretro-database. Both have been restored after the hack incident a while ago, so it shouldn't be an issue.
I've seen that there's a Python front-end for Mupen64 that has a GUI where you can select different cheats, but I'm guessing it just loads them before starting the emulator the same as the CLI you posted.
Okay so I got the following cheats for Final Fantasy 8 entered and uploaded into the files but when I activate them in the game menu the only one that actually works properly is the 255 AP and the Infinite Item Usage. All Items and All Cards doesn't work properly (All items gives me 100 potions and All Cards give me 127 of one card) I've checked multiple times and they are entered correctly, can someone please give me some insight as to why they aren't working? I threw the Master Code in there after the fact but it didn't change anything at all.
cheats = 18
cheat0_desc = "255 AP After Battle"
cheat0_code = "80078CE0+00FF"
cheat0_enable = "false"
Okay, finally after hours of tinkering under the hood I'm finally able to make Shining in the Darkness work with cheats without crashing RetroArch.
I tried changing the core emulator from pico to a different one and that didn't fix it. I tried renaming *.cht files and copying templates directly and that didn't fix it. I tried a different version of the rom with all three Genesis/Megadrive cores and that didn't fix it.
I couldn't seem to figure out how to use my USB keyboard to input cheat codes manually so I painstakingly physically drew by hand a character map using my controller and a pencil.
Eventually, I managed to figure out exactly where all the necessary keystrokes I needed were located and input a couple of codes and they WORKED! I was able to save this data as its own file using RetroArch and (crossing fingers) reloaded the game from scratch and successfully reloaded the newly made cheat file and it indeed works just fine.
I then input all the rest of the codes and the older ones and created a new file to load with the game.