Olive that sounds so hard for you :( For what it's worth, I did similar at the end of school (wasn't that long ago, I've just turned 22), for the same reasons, I couldn't cope with endings. I'm not good with change, and certainly not good with such dramatic upheaval as ending school.
I'm sure you know that the majority of BPD sufferers are abusive survivors of some kind, so it wouldn't be too surprising if she was a Borderline, given the trauma she's been through. Have you been able to discuss this possibility with her psych at all? Feel rather angry that the psych didn't deal with her needs if she was just told that she was leaving, and a 'withdrawal' process wasn't put into place, as is normally recommended with Borderlines.
Does she have plans for over the summer / in September? Making some steps towards preparing for those could be good; clothes shopping, equipment, applying for a part time job?
There are a few books that I could recommend to you if that would be of help, one in particular has a great section on coping as a loved one. Would also gently ask if you're receiving any support yourself, as this must be so hard for you.
The 'safety' steps; making sure she has no access to things she could use to injure herself (sorry to be so blunt), lock all alcohol away, medication in somewhere she can't access. Make sure if she goes to stay at friend's again, they're aware of the situation, and do the same.
Speak to the school as it may be possible for her to gain 'special circumstances' for her grades.
Much love to you.x