There's a procedure described in ADE's HELP function on how to do this.
Just press the F1 key when you bring ADE up.
On the Nook itself under settings there should be a "Unregister your Nook" option. This will remove both the Adobe and B&N accounts on the device, and remove all of the content synch'ed from B&N. If you are feeling paranoid about giving it away (how much of a friend is the friend ... ), the from the same place in the settings there should also be an option labelled "Reset to Factory Settings" - this will erase absolutely everything, including any updates that have been done.