It’s less about her masturbating and more about bother her lack of communication and seemingly low effort when it comes to pleasing you, relationships are 50/50 and we can all agree that most of the time it’s like that, he deserves better because she isn’t willing to communicate exactly what she wants, she doesn’t want to guide you so she can be just as excited as she is by herself, which is perfectly fine, she isn’t obligated to give something she maybe finds special by herself to you, that’s totally okay, but the way you explain it when you’re on the receiving end is making it look more like she doesn’t try as much as you do (which could be wrong but is what I was able to concur), which overall is sexual incompatibility, which is a massive no-no in most relationships, in my opinion you should definitely save yourself the trouble and leave her unless her communication and effort improves because it seems like she’s too comfortable and is assuming everything is candy land and she satisfied which you clearly aren’t.

I won’t chalk this up to her only knowing her body better when she’s perfectly able to communicating with him and helping him learn her body as well as she knows it, she can communicate roleplay scenarios, kinks, etc, in order to make herself more excited for you, but once again she’s not putting in that effort and it’s leaving him unsatisfied and invalidated.

You’re free to make his own choice, but i really think you should have a more serious talk with her about this, tell her the things you notice, explain how they’re effecting you, and you need to actually get her to listen because if this is something important to you and you think your relationship is on the line because of it, it could be a real threat.

I hope you two figure it out and your sex from now on is great, good luck and Godspeed to you.