I agree with sarahsweets. It's not too reliable. It's ok for an entertainment, but you don't have to take it seriously.

Your friend just happen to has NPD and get INFJ result. It just a coincidence, nothing more. There are no scientific articles or journal to prove the link between NPD and certain personality type. In fact, if you tried to google other personality type (ie. ENTP, etc), there are a lots of self-made theory about the link mostly from quora or other forum.

As for the cure, since it does not have the link, I think it depends on oneself, not based on 16 personality.