Be warned, this mod will corrupt your single player save files if you try to go to them with it running. You will need to create a backup of your minecraft executable (available through their DOSbox launcher) and change your client back to normal (sans mini map) before loading up single player.
Yea, it corrupted my first save. I loaded up my save and it was 19.89mb, as I traveled from one base to the next, it started getting EXTREMELY skippy (never done this EVER). I log out, log back in to load the save, and my save is now 19.3mb. It went completely unplayable, even with the mod uninstalled it moved as skippy as that, like it was trying to render blocks that no longer exist or something. :/
As to the illegal thing, I think as long as its not giving any help other than surface views and waypoints to set on the map, I don't think its a big deal. I think in all honesty, he should work out a recipe with the reeds to create a map that allows a mini map to be open anyway...
ALSO, this mod is great for all moderators because of the scrollable chat log. You can back track to see names of someone if they duck out and avoid a ban, or if they were saying something fishy in chat.