you need to install priiloader on your friends wii. if somthing was to go wrong, by holding reset whilst starting your wii, wii will allow you to goto priiloader screen> from there to homebrew channel> from homebrew channel to bootmii!!!!
bootmii will not load up directly on new wii's or if its installed as bootmii as ios!
p.s thers lot of benefits in priiloader like block disc update, enables you to play any region games, allow user to go homebrew channel etc. so google it and use it
My personal experience i have priiloader nd bootmii as ios and it new black wii on 4.1e, i did mess my wii up and yes i used my nand back up through priloader & bootmii which put the wii to back to how it was when i last made backup. requirement when doing a nand restore as ios is a gamecube controller(not wireless) without this equipment you cant restore nand on the new wii meaning any wii bought from or after 2008.
why its not recommended to restore because bootmii screen does not come directly wen wii start therefore you have no access to bootmii to restore but theres ways as i have explained. also system files is in bootmii as ios so if you restore and you have corrupted nand your will be bricked but the chances of that is very less and also it is better than having nothing as backup. as i said i have restored it once i had 100 percent success saved me from buyin a new wii.