Thank you for being a good friend—even if your help is met with some resistance, I am certain that she will appreciate it and that she certainly needs it.
In general, I think she has to just 1.) Say no that she doesn't enjoy this, want this, etc. and then 2.) Leave if he can't change his ways. It sounds like he won't honor her wishes anyway—but I do think it's important to start with a place of trusting/hoping/assuming that he doesn’t want to violate his partner. Though it sounds like he doesn't have much respect for her desires, I think she at least has to start there. She needs this for herself, too. I think before most people get resigned to leaving a relationship, they have to know that they have exhausted all possibilities. They have to know that they are leaving because they have no other option.
I am assuming that she won't want to report her husband—but of course, she should if she feels that is violating her. It's important to document this behavior because it's often repeated—and needs to be recorded.
There used to be a great group on Marital Rape based in northern CA—I’m not sure if it still exists, but I did just Google “marital rape” and rounded up many resources, so perhaps you could start by going through some of those resources and getting her connected to a group that’s close by to her.
I hope that helps—and I think your listening and trusting her will prove to be invaluable. The more you can nudge her to realize that she deserves better, the sooner she will be in a better place.