Another thing is that they have no history of lots of ex-lovers or overtly drinking. That's the most that makes me not 100% sure they have BPD. Is it still possible to?
My ex tried overtly drinking before, she didn't keep going because she always blacks out with bad hangovers later. As for ex-lovers, I know I was the 3rd guy she's engaged to and the 4th long(er) term relationship at her age of 25 (26 in October). That's 3 engagements in 7-8 years.
As for promiscuity, no. She was brought up in a conservative manner and we were each other's first physical relationship (I don't need to go into details about how I knew she was a virgin). Now, I wouldn't know. She could be starting to sleep with other guys now because that barrier has been broken down, and the fact she's mentioned IUDs during our breakup process.
Why would you really care for BC when you are not planning on sleeping with people in the near future?