Some things I ask before taking actions are 1) am I telling someone something they already know? 2) Is there any good that would come of it?
If you told her parents, would that give you the outcome you wanted? Examine the scenarios:
-they could say "yes, she has BPD, she has been diagnosed, we are aware." What does that get you, her, or her parents?
-They could say "no, we didn't know." And what does that get you?
If you are hoping a conversation with her parents will help you win her back, I'd suggest not even bothering because it will in no way help you win her back. If you feel you need validation, go ahead, but be prepared that you may not get it, or that validation won't have the effect you want. Will it make you feel better to hear directly from her parents that she has BPD? You already know she has issues, you witnessed them
Is that validation enough?
On the other hand, if your goal is simply to inform her parents your thoughts in hopes that they can help themselves deal with her better - by all means. I'm guessing, though, they have already dealt with enough crud from her and are already burnt out.
Believe me, I have thought about this many, many times, and ultimately conclude the same thing - that talking to her friends, parents, relatives won't in any way fix her or fix the relationship. All it would do is get something off my chest.
