Talk to your friend and tell her of your concerns, without mentioning the BPD. Even though she doesn't have BPD, she's bound to feel the same way to some extent. Then, work out a strategy to keep in touch. It's much easier to keep in touch than it used to be. There's instant messaging, video conferencing, texting, and a million and one other ways to stay in touch that didn't exist 20 years ago. If you have a distinct plan in place on how you're going to keep on touch....make a schedule on WHEN you'll do these things not just how, maybe it keep your triggers down, and make your friend feel a little better about leaving too.