Then I think you need to disengage, she doesn't mean it when she says she's goingto leave him, so stop responding to it. Do you say "I think you're right" and start helping her plan it? Then it sounds like she's saying what she thinks she should say, not what she wants to do at all. Even when she's saying it, she doesn't really want to leave.
To you, it would be impossible to live like that, but you need to accept that she's actually rather happy with her life. That might seem insane, and when it's bad, she complains, but it's not that bad to her. It's actually OK most of hte time. She can live like this, to her it's preferable to the alternative, being without him and starting again.
Sadly, you have to sit back, listen but don't emotionally believe in her, don't try to solve her problems, listen to her rant, don't offer any solutions, don't suggest she leaves, just listen, while in the back of your mind, repeat to yourself "when she doesn't want to live like this, she'll leave."
You've offered her practical support, she knows how to leave, she's chosing for her own reasons not to. Don't believe her when she says she's leaving unless she's turned up on your doorstep with a bag of stuff. Before then, listen, don't advise and remember while she's saying "this is it" that it's not, she doesn't want to leave, or she'd go.
It's like drinking or any other adiction, until she's ready to give him up, there's little you can do. I've heard many drunks tell me that they are 'never drinking again' or are quitting, it doesn't last until they hit their personal rock bottom, which is a different point for different people.
She might never be ready to give up on her relationship. You need to decide if you can be her friend knowing that this might be every conversation you have for 30+ years. And remember, you don't have to be the person who supports her.