I have a nearly 28 year old narcisstic daughter (undiagnosed other than by me and our entire family of course) because WE are the problem, not her.....ever. She is selfish, constantly takes pictures of herself (she is pretty but come on.....it has become a joke...) - she has a nearly six year old son who she uses perpetually as her PAWN - manipulates everyone she possibly can, hangs out with a bunch of lowlife losers.....Keep in mind, she was NOT raised like this, both her father and I sent her to private schools, she graduated sigma cum laude from college and this behavior (although in retrospect there were earlier signs) has just become prevalent the last two years....also she was in nursing school for a bit and was a total tee-totler (no drugs no drinking) - she has done a complete 360 on us and I don't know what to do to help her, she has so much anger towards me and actually told my sister and I that the reason she treats her son the way she does is because "she was too spoiled and she doesn't want him to turn out like her???" SO HER SOLUTION, party, party party, quit two great jobs, let a couch hopping bum five years her junior (with 3 kids???) move in with her and do drugs, party and fight...She caught him cheating, told everyone and then took him back after she confronted the girl who slashed her face, she is so sick, how do you get them to realize that they have a problem and how I get her some help? I am in counseling, my primary concern is my grandson.......this is affecting him, he lies for her to protect her.....it is so sick.......and dysfunctional...
She may often be wonderful to her son - when his presence confirms her image of her wonderful self. If he does not make her feel good about herself she will be vicious. I survived (and for years adored my father) by being "perfect" and showing how much I admired him. When he turned on me it was devastating, but by then I was old enough to cut off ties with him.
If your grandson lies, you do not have to believe him and he will surely benefit from hearing you say "I know that is not true, but I bet you wish it was! You really love your mom." This is the truth and if said in a loving way will help him feel cared for and respected for his youthful efforts at managing an impossible situation.
She is likely not to go for treatment - unless she can be convinced it is to deal with problems she has that were caused by others (she may enjoy talk therapy since she can change the stories and fool the therapist and herself).
I am so sorry for you and your grandchild and send thoughts and prayers to you.