I've been friends with someone that has BPD for over 10 years now. She only recently discovered that she had BPD but hasn't gotten treatment yet. There is always one terrible unstable decision after another. The way I interact with my pwBPD is with endless patience and an acceptance of her decisions even if I don't agree. I do have gentle discussions with her about what she did or is deciding to do and I always give her my point of view and remind her every time that it is how I feel and it in no way can impact her unless she decided it does. I've accepted that she can blow up and get mad at me. I accept that she will make bad decisions and talk her through it even if she won't take my advice. I never say I told you so and I help her through as many bad decisions as I can. I have firm boundaries in place that allows us to have a healthy friendship. We have been friends long enough that she knows my boundaries well and doesn't push them. I hope this helps and good luck!