Most don't have the awareness or the incentive to grow as human beings.
My mum, similar to others here, could not cope if she thought she was anything less than perfect and that I wasn't the problem. The more she tells herself that the more energy it takes to keep up the lie. She would find it even harder to be around me nowadays (we are NC) because I show glint of the uncomfortable truth now and again, and if she had to see this clearly, I think she would kill herself.
So I let it be and hope she lives the end of her life reasonably well. Don't forget that living with a disorder like NPD is pretty horrible, karma in itself. I think it's better as pp have said not to waste your energy. You'll only firm up the beliefs they already have.
With men in relationships, I also think it's better not to say anything once you realise it's actually over. As they navigate their way through other relationships, maybe, hopefully, they'll start to get an inkling that they could be, at least part of the problem.
I once read that when you die, you see everything clearly from a point of expanded consciousness..