Some friends of mine have a central unit that has failed: the compressor. They were giving permission to us another unit from a neighbor who was moving and selling their house. My question is I was trying to disconnect the pipes to their unit and the nut on the pipe won't disconnect. It will unscrew back to a point and stop and won't come apart. Is there a trick to disconnecting the pipe? Does the freon need to be discharged first before it will dettach?

STOP right were you are. This is not a DIYer project and you can injure yourself. Freon must be pumped down with special equipment. Illegal to vent to the air and can burn you, besides the freon on the other unit can/will be under pressure in many cases. You will need a pro to dry and pump down the unit before refilling it with the correct feon so call and ask for a bid on the whole project.