This is the person who finds out they have devoted their life to a sham or that their hero has feet of clay.
And the treatment technique for borderline personality disorder called Dialectical Behavior Therapy is well established and heavily evidence based. Sees self and others alternatively as all-good and all-bad. People with borderline personality disorder has a persuasive pattern of instability in interpersonal relationships. Narcissistic personality disorder: Egotistical, arrogant, grandiose, insouciant.
Leaving aside the question of whether you should stick around, and assuming instead that you see some value in this person, you may be wondering whether they are exhibiting signs of a personality disorder.
And if you think they are, you may begin to speculate as to whether the person is a narcissist or a borderline.
Under serious stress, both types decompensate and can become essentially non-functional.
With the deflated Narcissist this can take the form of extreme rage, suicidal despair or sociopathic acting out or some combination of these.