From an article here.

Can narcissism be “cured”?

Maybe. It depends on the skills of the clinician. Most behavioral clinicians have difficulty knowing how to work with one. For an extreme narcissist to be “cured” he must want to heal and be willing to admit he is unhealthy. For most extreme narcissists, that is simply asking too much. For an extreme narcissist to want to change there must be a gigantic and earth-shattering series of events in their lives to break them of their grandiosity, extreme selfishness, entitlement, and self-righteousness.

Should an extreme narcissist be willing to be helped, the clinician must be skilled enough to find the wounds of his inner soul, help heal them, and replace them with healthy self-images and patterns for relational dynamics. The only kind of extreme narcissist that can be cured is a broken one.