I know and completely understand what you and she are both going through.
Getting her help was the correct step for you to take. If she can get involved with the DBT, it is the best therapy for her. She needs to find the right doctor/therapist for her and the first one she attempts, may not be right. Moving and changing to find the right fit is necessary.
You may not have a choice as to whether "YOU" decide to stay and help her. She may ultimately push you away (this is the mostly likely scenario). And you won't have a choice about it, which is probably why you are not tin a relationship any longer.
You probably no longer are interested in the romantic relationship because of the constant ups and downs and rollercoaster activity you have observed over time from her. But this is her deepest fear and if she was originally the first one to pull away, she knew this about you and sensed it. It is said that people with BPD have an extra sense about the feelings of others, a deeper intuition.
Ultimately she fears being abandoned, which is what you have done to her, in one way. But do not feel guilty, in her mind, everyone has done this to her. Even if you want to stay to help, if it is not really what she wants or even if it is, she will find it hard to stay friends with you. It has become a self-saving mechanism to push people away before they can hurt her and she is now at the point where she can no longer help herself. It is a vicious cycle. No matter how much she wants the love and to have a relationship, she cannot maintain it.
Internally, she is an emotional wreck. Depression is the worst part of it. Self-loathing and never feeling up to par. Never feeling like she can ever be the person that someone wants her to be but worst of all she can never meet her own expectations of herself. Eventually the pressure, becomes too much and she starts acting out and pushing people away. Suicide is a huge worry in these stages of depression. She needs to be monitored by a Therapist and a Psychiatrist.
She will also probably notice (you may too) that her moods come in waves, usually about every 3-4 weeks it gets worse, then the storm clears for a little while and then in 3-4 more weeks the wrath of the storm comes back. Her moods and depression are based on her menstrual cycle as well, often heightened in BPDs.
Please note that it is crucial for her to continuously try to find the right combination of medication. Anti-depressant, mood stabilizer, and possibly anti-anxiety meds might be necessary. She or someone close to her needs to help monitor these as she won't necessarily notice the side effects or how they are truly effecting her. No one drug is right for every person. Some people take years to find the right combination of meds for them, and then as they age, they may need adjusting. And what might work well for two or three years, may all of a sudden not be right any longer and needs changing.
If you cannot help her, please try to find someone who can. Professional help is the best for her. Keep in mind, she may be resistant. She needs to be shown the light of this situation and how certain people helping her are the best thing for her.
And in case you may be wondering why I know these things about BPD, I was diagnosed with it myself 5 years ago. I have been dealing with it for my entire adult life. When I was 23, I was raped. This event started a spiral of unworthiness and self-loathing in my life for years and continues to get worse as I age. I am now 47 yrs old. However, seeing my two doctors once a month and staying on my meds does really help. I served in the military fro 25 years, which according to most people was a major miracle that I was ever able to accomplish as most BPDs really struggle with any normalcy and holding jobs. I also went to college and earned two B.A.S. degrees.
There really is no recovery from BPD, only finding ways to cope with it. But I have found that in doing things that keep me busy, help me progress, and better myself, do help to clear away the ghosts in my closet and help me feel more confident in myself. It is a battle I face every day, as it is so much easier to go to my bedroom, close the blinds, crawl into bed, and hide from the world.
Also, my pet is my best friend and I find that he is a wonderful support system for me. Unconditional love with no judgment. Getting her a pet might help relieve some of the stress and anxiety, but it needs to be the right situation for both her and the pet.
Good luck and I hope things go well for both you and your friend.
Both, contact and lack of are likely to trigger negative emotions.
Going through a third party is counter productive. If you want to maintain contact, then do it, but set boundaries on the relationship at the beginning.
How would avoidance be helpful? If you want to support her, support her through the process.
I have found DBT unhelpful. DBT can do more damage for some individuals.
I have been listening to some material from Kelly Brogan. There is literature to suggest that nutrition plays a big part in emotional regulation.