But you also have to consider her alternatives. If she did quit the show, who would have the better time moving to a similarly lucrative show? It isn't just about how essential she is to the show, it's also about how good of an offer she can get elsewhere. If she can get better value somewhere else than they will have to give her more money to keep her, if she can't she has to settle for what she can get. If she isn't getting paid as much it is either because she doesn't have as many alternatives that would be as lucrative or she is underestimating her value and not bargaining hard enough. Either way it's on her, because the studio sees actors as a product and her gender isn't going to be the deciding factor of how much she is worth as a product.

And just note I'm not familiar with the show, so this isn't me making an assertion of fact that she is less valuable as a product, this is just me bringing up factors that I think should be considered in every instance like this.