A friend of mine has an N3 w/o any software. Can he use my D1 discs? Are they universal among the D*/N* models?

Should be able to since the mapping software is pretty much the same between the 2.

I doubt it since the N3 disks and the D2 disks are the same. Putting the D1 disks in the N3 would be using older software than it shipped with, and that usually doesn't work on Pioneer systems