Don't let them have any exposure to them. If they do then you must utilze very firm boundries with them. If you give an NPD an inch, they will take a mile. Be careful, proceed with caution If they are under 18 do what it takes to physically keep them away from the N. If they are of age all you can do is tell them the truth and caution them to avoid the N and be there for them when and if they should get mixed up with him. If you try to be too forcefull and pushy it only makes them avoid you and run to them.