The person prolly isn't a clinical narcissists only a psychologist could diagnose as such. When you do find one though it's a very tough case.
To be honest one of my current psychologist and I discussed the situation of narcissist therapy in detail. I dated someone who fell somewhere in the empathy disorder spectrum. The person wanted me to announce them to the world and due to my socio-economic situation it would result in what little family I have left disowning me. In general psychotherapy can teach someone who is thus sick how to behave in a manner consistent with the norms of society but there remains some question of current methods ability to remedy the disorder in a way that the changes cause the universe which that person experiences not to revolve around them.
I suggest an appeal to ego might work. My appeals to morals, emotions and logic did not work. When I bruised this persons ego centered around them doing something obviously stupid that was causing them physical pain they responded well. The person hurt their foot after we broke up and the doctor would not give them pain meds because they would not ask for fear of becoming addicted. They had however discussed their affinity for Mary Jane so I offered to buy them some pot and they refused. It's legal here but could not afford the trip to the special doctor or the pot itself. I swore fine and when you want your nose reattached for you seem to have your mind to remove it I will pay the plastic surgery. They asked what do you mean and I explained the concept of Cutting off the nose to spite the face. Then my help was accepted. So I would frame the argument around self betterment. Do not attempt to point out fallibility of the individual to self help. They love gifts. Maybe buy a book that deals with psychotherapy and includes something that resembles their problem. (Mean Mothers: Overcoming the Legacy of Hurt if they are female and maybe the lesser The Emotionally Absent Mother: A Guide to Self-Healing and Getting the Love You Missed
If male a lot of problems are so inherited from parents and learning about their parents flaws may help them self identify) A lot of important people in the world bare some of the dark triad traits and have learned through therapy how to be more successful and influence people.