How can I help a friend who is on a very destructive path?

By getting them some help. You need to define destructive, though. Are they drinking too much or trying to cut themselves? Anything that is physically harmful to a person is not of wiccan rede or idea, so if your friend has the wrong idea of what it means to be a witch, you need to get them some immediate medical attention. If they are simply spouting wiccan rhetoric and have no idea of what they are really doing, ignore them.