You could also take a look at
This Thread about how to make "favorites". You can also use the method explained to map out a custom route for any reason.
If your friend has a coordinates display, you could always have them fly into the nearest flight point, then tell them which direction to head on a road that comes closest to where you're both going, give them the coordinates to turn off the road, and the coordinates for the entrance to where you are going. No need for a complicated routing scheme, just a set of coordinates and a direction to go from the nearest flight point.
I remember doing this to get to Wailing Caverns when I first started playing WoW (about 4 years ago now, and before I got Carbonite), and I had no idea where it was. My son did this for me and I easily found it.