there is a way to convert a NAND from one wii to another but it's not that easy...
check out this thread:
[
forum.wiibrew.org]
only bushing is offering this at the moment and it will cost you $10.
you also HAVE to know the encryption keys of the wii which is supposed to get the working converted nand backup.
so obviously this only makes sense if you already have a bricked wii and a way to reprogramm the NAND; if the wii of your friend is not bricked, you can simply install bootmii and make a backup.