It depends on numerous variables. The narcissist's reaction depends on WHO does the diagnosis. He will become cold and aloof and disconnect from the diagnostician, the former having lost its supply source status by daring to make such a diagnosis. The reaction would be no different to a verbal confrontation unless intimidation is involved. If intimidated, the narcissist will recoil and become submissive, overly sentimental, dependent and idealizing.
All generalizations are false.
I discuss the narcissistic couple in one of my FAQs . Narcissists can be happily married to submissive, subservient, self-deprecating, echoing, mirroring and indiscriminately supportive spouses. They will also do well with masochists. The Narcissist makes use of this naivete and exploits it to his benefit. The natural protective mechanisms which are provoked in normal people by love - are cold bloodedly used by the narcissist to extract yet more narcissistic supply from his writhing victim.