FlashMe installation is indeed just as simple as running a .nds file and following the prompts. However, I don't think it will really help you for what you're trying to do. The closest thing to what you want is "Wireless Multiboot" but that was for sending a single *.nds file from your computer to the DS via DS Download Play. This only worked on specific wireless cards, and you needed to have FlashMe installed if you wanted to send over homebrew files; without FlashMe, you could only send officially-encrypted DS demos.
Wayyyy back in the day, someone managed to get a partially working WMB host on the DS itself so you could send the .nds file from your own DS to the other one, but that was way before the days of DLDI so it only worked with maybe 3 slot-2 devices.
A better idea would be to just put your slot-1 device in your friend's DS, launch the homebrew file, and eject the card. The program will stay in RAM and keep running until he turns off his DS (erm, at least I think it will work. I don't think the program will freeze, but my memory is hazy). This will have the exact same limitations you would have when using WMB, namely, the .nds file must be completely standalone and must not require filesystem access to function. Since a large majority of homebrew nowadays uses filesystem access, this will severely restrict the available titles he can demo, but it's better than nothing.
EDIT: Regarding the FlashMe installation, I've heard the number stated as 3% to be sure you can recover if needed, with an absolute minimum from some people saying 1% is enough. I'd be more concerned in making sure the emergency FlashMe recovery mode can boot your specific slot-1 card, though.
