Diane D. is right -- making a change would frighten her and she knows it. Being stuck is the reality she knows best and the reality in which she is most comfortable -- yep, comfortable, because she probably defines herself by her issues. She sees herself now as the woman who is in a relationship that is not working, but the idea of either letting it go or changing it is probably terrifying to her. It's a pity that therapy for so long has not helped her make changes. Bur if therapy can't do it, neither can you.
So what do you do, as the friend or relative? Well, if you like spending time with her -- if there is a personality there that you enjoy most of the time, or if you and she share some interests that take her out of her own head and make her fun to be with at least at times -- you focus just on that. You maintain a relationship by doing shared things that interest you both.
But you do not have long sit-down chin-wags where you dig over and over her past, her marriage, her divorce, her current relationship, her being stuck. Do not let her drive every conversation toward her own issues and past (if that's what's going on).
If she uses you as a sounding board but the things she is saying have been the same for years, it's past time to distract and deflect. When she starts to say for the millionth time, "This is the latest thing that Significant Other did that makes me unhappy," you need to listen only briefly before you change the topic. If she notices, you need to be able to say, "I do want to be here for you, as the saying goes, but I also want you to be able to think about other things and leave that behind." Bring up the shared interest. Go on outings rather than sitting around chewing the fat you and she have chewed for so long. Watch a TV show together that you both love, and talk about it. Go to a concert or play or other event and talk about that. If she drags her past into it every single time, steer her away.
But if it all becomes a huge effort, then tell her: "I know you're not happy with Significant Other but I don't see things changing. I really like spending time with you doing X and Y, and I really don't like feeling Significant Other and Lousy Ex are always hovering over our conversations. I want our time time to be about our friendship. So let's go get a drink (or whatever) and you can tel me about (another topic free of her ex or her SO)."