OP I would be wary of a borderline diagnosis or EUPD (Emotionally Unstable Personality Disorder) because of the stigma. I don't think you realise how much stigma there is around this diagnosis, especially given how some therapists won't touch it. It's now been changed to EUPD because of this as it's been reframed as a mood disorder. You can read more about that here.
I wouldn't be so fixated on that particular diagnosis, personality disorder diagnosis are pretty contentious for a start and one therapist may disagree with another on the diagnosis. They don't take environmental factors into account. It's difficult to diagnose and women are often diagnosed with it more so than men. It's very easy to say that an angry, traumatised woman is 'borderline', rather than reacting with rage to trauma or other events in her life.
Others are correct in that the brain hasn't really settled until late 20s and to stigmatise a person with this for the rest of their lives, when they could change and settle down, is not the best idea. Also, Borderline is often misdiagnosed like I said above, for various reasons, one of them is trauma. If you look up C-PTSD, this is often misdiagnosed as borderline.
These rages, since you talk about anger management could be hormonal. Did they start around puberty? Some women are completely enraged around the time of their period and that settles down. Since it's hormonal, there's nothing they can do about it but there are supplements such as Primrose Oil, that can help. Rage isn't the only factor in a diagnosis, identity diffusion is a huge component of this, which is unstable self image ie the person doesn't know who they are as a person and their identity is constantly shifting.
Finally it's easy to look for a diagnosis in order to take away responsibility for your behaviour. Even if she does have a personality disorder, she'll have to learn to manage it for the rest of her life and will have to take responsibility for her behaviour. Good therapists won't treat a 'diagnosis', as it's easy to frame a person within the diagnosis, which isn't helpful. You might find this helpful to further understand that.
DBT is available privately and it is possible to recover, or at least recover enough not to be beholden to the disorder. There are specialist clinics for personality disorders and these are available on the NHS as well as privately. There are also forums, online groups and she may find some medication helpful.