IMO, DBT would improve empathy, b/c in Stage II of treatment, the goal is moving from being emotionally shutdown to experiencing emotions fully.
In Stage IV, the goal is moving from incompleteness to completeness/connection.
If you visit Marsha Linehan's website "Behavioral Tech", there is an abstract there that though somewhat clinical (b/c it is written for professionals, I believe), is still helpful in some respects for the average person who is not trained, in gaining an understanding of the goals and principles involved with DBT. I did not see the actual word "empathy" used, but it stands to reason that as patients make progress emotionally, this could very well be a welcome byproduct.
Empathy may never be experienced as fully as it is for nons, but IMO, it would at least improve to some degree over what it was, even if that improvement were limited.
I have learned recently that a casual friend of mine is a confirmed pwBPD. She is also in DBT. She told me she has been searching for the past 20 years for a treatment program that would help her, and that DBT is what is finally making the difference in her life.
She still has a way to go, but she has already made alot of strides. She seems to have empathy for her own mother whose physical health is suffering, and she "seems" to have some level of empathy for me in my own personal situation, though I still see her empathy lacking when it comes to others we both know. How much of that is due to her own BPD, and how much of that is due to the conflict that exists between her and these other people remains a question.
Even though I am a non, it is very difficult for me to feel empathy for someone who continually crosses my boundaries and who frequently tries to pull me into their own conflicts, or who habitually tries to create conflict with me. So on one hand, I understand her lack of empathy for certain people who can be historically problematic. Just b/c I have learned how to effectively deal with these people, does not mean she has.
For me, the closer I am to someone romantically, the harder it is for me to feel empathy, if my partner (currently, my PDH) is the one who is habitually problematic. In the absence of positive change, my empathy eventually goes out the window. And when I see someone else who isn't doing anything to help themselves, it then becomes difficult for me to feel empathy for them as well. I have to detach from them to protect myself, but when I do, I stop thinking about them, and my feelings also diminish as a result. Easier to do, when you don't have that much contact or a close connection with someone to begin with.
I guess the problem is if someone never has the ability to feel empathy at all, no matter the situation, which seems to be a common trait of pwBPD. Otherwise, I think it is normal for it to ebb and flow, depending upon the situation, where we are at emotionally, and the level and condition of our relationship with the other person. When you are dealing with someone who has consistently proven themselves to be untrustworthy, at some point, you have to detach to very high levels, but when you do, your empathy dissipates as a result. That isn't b/c we are defective - it is b/c we are dealing with someone else who is defective, and it becomes the only way to protect ourselves. That being said, sometimes I feel my empathy returning for my PDH, but when I do, I work hard to bury it, b/c if I don't, I might end up caving to him.