First of all an empathy deficit is a problem in many illnesses, substance abuse, depression, psychosis, borderline disorders, bipolar, its quite a list. In the same way a "cure" for NPD is also a long list of stuff and empathy, relevance-wise, really is quite low on that list. More prominent is that the patient doesnt become psychotic for example or aborts treatment in a depressive, possibly suicidal state.
Secondly, pwNPD dont actually lack empathy. The existing studies show that pwBPD lack affective and cognitive empathy depending on the degree of their illness. In NPD its only affective empathy thats missing and also not necessarily totally, while cognitive empathy is usually intact if one isnt on the psychopathic end of the Kernbergian spectrum. I remember one of the later studies on CNPD for example actually describing pwCNPD as "interpersonally warm".
Frankly your misguided fixation on empathy seems silly and egocentric.