Stuck in a miserable marriage...... no idea what to do....

My mother's husband is, for lack of better word, OBSESSIVE. and unbearably obnoxious, to everyone in the family. He's constantly hovering over her, following her around the house making stupid ass comments, and just behaving like an obsessed dog. He very well knows that it bothers all of us, but it's extremely apparent he simply doesn't give a shit. I've lost count on how many times she's told him that she's not happy with him, he's unbearably clingy and sexual 24;7, that she wants to divorce him. but the next day he's back to same ways, as if he was wearing ear plugs when she basically told him that she wants nothing to do with him. She would've served him divorce papers long ago, but they had a child together, so due to the whole custody shit, he'd get her on weekends; you know the drill. And she and I both know that she wouldn't be safe in his care. So it's like she's trapped, I can't bear watching her miserable in this marriage. it's tearing her apart. what should I do? What should SHE do? She's absolutely f***ing miserable, I can't take watching this anymore. Thanks