As someone who has dated someone that (I'm fairly certain, but not officially diagnosed) had borderline personality disorder (e.g. had all traits but the suicidal ones listed in the DSM) I personally wouldn't want to be involved with someone who was like this again.
The current though of those that put together the DSM is that personality disorders are lifelong and unchanging. (There are some more fringe or newer schools of psychology that think fundamental personality change IS possible, but it is worth noting that it is still difficult and the current official position is that it can't be fixed.) That being said, borderlines are notoriously the most frustrating of the personality disorders even for psychologists and psychiatrists to work with.
From my personal position, I would state to your partner what you need from her. If you need a certain amount of her time or energy, and she is not providing this to you because of her borderline partner's drama, then she is not being a good partner to you and is not meeting your needs. As someone who knows better than to reason with a borderline or to expect them to show maturity in their relationships, your partner needs to learn this lesson.
Unfortunately, it took three tries at romance and then another at friendship with my borderline before I finally learned my lesson. He would vacillate between being my best friend and my worst enemy, and my fiance and I were constantly having to drag him out of his own mess during the last try at friendship. We had to call an ambulance for him when he though he overdosed on a drug, and then two weeks later his girlfriend (who wasn't going to help him at all with the drug thing) got mad at my fiance over something really stupid and then he flew of the handle and told us that we were terrible people in his life and he never wanted to see us again. A few weeks later, we were all friends again, that is until he got mad over another something small and then threatened to call the cops on us.
Basically, unless she is actively in treatment and making progress toward healing herself and learning better habits, she is never going to be capable of having a healthy or stable relationship. I understand better than anybody how addictive their personalities can be for someone who isn't experiencing it. She isn't being disingenuous. She means everything she says and does. But she is a human wrecking ball and she will probably continue to be one for the rest of her life.