It is the same mindset that people get when contemplating the divorce from "cheating" spouses. In other words, they feel like it must be to their advantage if they can show they are the "wronged party" and that the other person has "sinned" by doing whatever it is that they are trying to prove that this person did. IF this OP went to divorce court with recordings, transcripts, etc of verbal abuse, it would not be surprising if a judge simply refused to hear it. And no responsible attorney would permit a client to wander off down this path.
But I am hearing that this person doesn't want a divorce. She merely wants to win the argument that the spouse is verbally abusing her, NOT that she wants a divorce. And of course, having shown other people what a bad person he is, and how bad he is treating her, she fully expects that this will magically stop the behavior.
I strongly suggest counseling for her, and NOT marriage or couples therapy, as this person would probably do what I have seen other spouses do in these types of situations, sit there and refuse to say anything helpful in her own defense. I also suspect they've had some marriage therapy, in which as she says, the husband angrily and adamantly denies being verbally abusive. But this is not really a legal problem, as the illegal recording of his verbal abuse will do nothing except...well, exactly what could it possibly accomplish? We all agree that it's not going to work for her to gain any type of advantage if there were a legal proceeding. And her magical thinking is that the minute she definitively proves that he is verbally abusing her, he's going to stop doing it, right? She will somehow win the argument, and he'll immediately change the negative behavior. Nice idea, but not worth all the trouble and strain of making such a recording.