Welcome to the group, sorry to hear about the difficulties arising. Are the symptoms relatively newly on-set or existing from puberty or some other age? Is she hesitant to speak to anyone about the symptoms, or even refusing to acknowledge them at home, or possibly open to seeing a professional? There's no "magic" to a BPD diagnosis, since unlike something like schizophrenia there isn't really a certain medication for it - it's all just treating the symptoms themselves. So if there's anxiety, or depression, or violent fits, delusions, black-outs, etc. With or without a diagnosis, many people here love the DBT suggestions by Marsha Linehan (on Youtube) on how to communicate with a BPD loved one to avoid "triggering" them as often.
Has she needed hospitalization at any point or been in serious trouble? I'm sure it seems like forever, but the good news is that this phase is typically the worst of them and as they age they lose some of the frenetic spasticism in something like 80% of cases. Is she generally able to hide her symptoms from peers and outsiders, or does it seem like most realise something is a little...off?