My Nmom went to therapy for almost two decades (or maybe longer actually) and her therapists did not a thing for her. She had them snowed. Even the best of therapists can't really do anything to help Ns unless they are doing something to help themselves. PDs are hard work. She'd probably just placate the therapist for show and be a bitch at home anyway.
You might be best to tell him to stop expecting him to change and focus on changing himself so he can be happy and free.
As you you, I will give you some words of wisdom from my own experience - Ns don't change. They don't want to change. The simple reason why? When you have control of your environment and everyone in it and that makes your life easier and gives you the twisted satisfaction you seek by convincing yourself they do it because of your awesomeness (whatever said awesomeness is delusionally believed to be) would you change to make them feel better or would you just keep doing what you are doing and probably mess with them for kicks cuz that's what you do as an N?
Seriously consider that question. I have learned that my Nmom wants to be how she is because she gets more out of it than trying to change and I have learned that I don't like her N behavior and that she will never change that core part of herself no matter what. She won't even acknowledge there is a problem. She lies and says she will behave differently than he selfish person she is (and honestly, she's almost not very N like in many ways compared to the stuff I read on this forum. She's actually almost awesome compared to what I see here. Her worst days are in the distant past of like 6 years or more ago), but she never changes. She has mellowed but that had nothing to do with therapy and more to do with me getting so sick that I shut her out for four years while living in the same house and her learning that there really is nobody else for her to turn to so she has to be on her best behavior, only now she uses illness and health issues as her manipulation so I do it right back to bow out of her games. And this is how it is with Ns. You can think they are better and they might improve in some ways but they will ultimately just find new weak spots and new tactics to use to exploit people. They are HIGHLY adaptive in that way, so we have to be either highly adaptive in return, disengage, or be extremely forceful in our boundaries.
You could see her going to therapy and she might learn things from the therapist and USE THOSE THINGS to find new ways to manipulate you in ways you don't right off comprehend and before you know it, you are back in that position of having get out from under the games and cons and lies and manipulations. Save yourself the favor and do not bother. There can be no compromises with an N. If they SEEM to compromise they are doing it for self preservation and they will simply change their tactics as my Nmom has changed hers.
Be his family and disown her. It's for the best really if you can do it. Think of it as this - if you do love her, letting it go and letting her go is the best thing you can do for both of you because otherwise you will keep seeing the worst in her and you will keep feeling bad about all of this. With time and space you can let go of it and negative feelings and experience can be soothed.
Just don't trust her that therapy will help or that she would even go to it. Ns do NOT like to be told what to do and if they ever are forced to do something they don't want to, it just means they will rack up a whole lot more spite against you down the line.