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Hello and thank you for using JustAnswer. To diagnose someone with Narcissitic Personality Disorder (NPD) the individual would have to display symptoms of grandiosity, lack of empathy, failure to take responsibility for one's actions, pathological liar, manipulative and exploits others, can be arrogant, envies other peoples successes, and/or requires excessive amount of focus and admiration. Here is a good link on NPD that explain the diagnosis further for you:
As for how to deal with someone who has NPD, nearly every psychologist will tell you that an individual with NPD is unlikely to change and therefore you have to maintain good boundaries and consider separating from said individual. This would require minimal contact and to focus on yourself more instead of the other person; so overall the best advice is to separate from this individual as soon as possible. No matter what coping strategy you have, it will only provide a temporary solution as the NPD individual will adapt and exploit; remember it is not you that must change it is the person with NPD since they have the problematic behavior. But because they are unlikely to change it is better for your well being to consider escape and moving forward with your life.