Hahah this is quite the ordeal!! Welcome to the forums.
If it was a 100% run, unless there’s a name involved with the save file, you can just download a complete save file online. He will never know.
If you can remember the process you took restoring the OS, post it here. There could have been a missing step. Also, if you remember what version, write it down too.
Naturally, I am biased to recommend my own image; DEOT+ v2. It even has a boot up sound of Megaman’s Buster charging, and a few other bonuses. There are some instructions on how to flash it in the first post of this thread:
If he’s still using the original OS, anything made that’s 0.5 or newer would be a significant upgrade.
If you did write the image properly, or if you want try and boot up the gameshell with your original image, ref doing it with the USB power plugged in, and also try pushing the hidden reset button. Check out this thread for a similar problem.
Good Luck!