Empathy is Empathy regardless of who it is to. Even you just asking this question is wonderful because it shows that you want to give empathy to someone who is struggling. I think the best way to figure this out is to ask the person who is struggling, because everyone is different, but also there are other things you can do to help support anyone struggling from BPD specifically. You could double check on them if you feel you are stable enough to allow them to lean on you for support. Just remember that everyone with mental Illness are different, and Whay helps somebody may not help someone else.