It's really difficult to manage people with bpd because it is a disease that is so very difficult to treat successfully. They are so manipulative, have the worst mood swings and can be violent and self destructive. All you can do is remember that their disease really controls a lot of what they do and be understanding but to a point. You do have to set boundaries. Provide an environment that is as stress free as possible and don't pay attention or argue about things that really don't matter. Put your foot down when it does matter, walk away and follow through knowing they are going to react and sometimes in a very dramatic way. Being a friend to someone with this disease sometimes has to be about taking care of yourself, keeping yourself mentally healthy because often there is nothing you can do to help them or change their behavior. Encourage meds and counseling and be very supportive and encouraging when they are doing well.
Does Darth Vader/Anakin Skywalker in the entire Star Wars series count, and for them to be fixed, no you don't have to have them save you from an evil emperor, just let them have therapy or some medications proven to decrease their symptoms.