For what it's worth, it's not clear whether she's suffering sudden rapid-onset attacks of self-loathing/suicidalism/violence/cussingAndThrowingAboutOfObjects, etc - but if so, in our experience Alprazolam is the most effective drug by far - but because it's a Benzodiazapine and commonly sold on the streets and stuff, doctors nowadays try to avoid giving it to anyone "because there are better drugs now!".
Worth trying to ask about it by name.
On the non-medicinal friend, if you're her only friend I'd say to take it seriously and again, anecdotally, keep in mind...
The "normal" person is more grateful and happy to receive a rose from a loved one 12 times a year, than a dozen roses once a year. But the haltlose rate in BPD patients mean they're often the opposite - they lack a firm sense of their current place in any timeline and live off a hyper-artificial mental image they construct. So it doesn't matter what you do, it'll be forgotten in 3 hours and they'll swear you do nothing...then 3 days later cherish it, 3 weeks later claim it never happened, 3 months later swear it was the greatest thing anyone ever did for them, etc. But therefore something "big" is much preferable to them to a lot of small kindnesses.
In short, think of something so gaudy, tacky and melodramatic that you'd roll YOUR eyes if someone did it for you...but to them it's the adult version of going to Disney and getting $20 of free tokens and having cotton candy,
. They'll complain some of the time, same as with anything, but the other times when they remember it fondly it will have a MUCH bigger impact than asking them to remember a thousand small kindnesses and mercies you showed them.
