Hello everyone, I hope it's okay for me to post here.  One of my best friends has BPD and a series of events spanning over the past several months are finally coming to a head and I want to know how best to support her.  I'll try to keep this as concise as possible!

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I'll call my friend V.  V began dating my best friend, B, about five years ago when he lived out of state.  They moved back together three years ago, got engaged the year after, but she was unhappy before they moved.  She moved out of a very abusive household to move in with him when she was 18, and loved (and I think still does love) him very much, but wasn't attracted to him any longer.  She also feared moving back home so she decided to try to just make it work.  Because she wasn't super interested in him sexually or romantically, she suggested opening up the relationship, which is something he was never interested in, though he did try to compromise a few times with pretty disastrous results, namely, her growing frustrated with boundaries and simply cheating.  They made up several times, as it goes, but it was an ongoing problem.  The last poly attempt was with one of B's closest friends, and it quickly lead to V and B breaking up.  V was still very interested in the new person, P, and wanted to continue a relationship with them post breakup.

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While V was first getting involved with P, she left out the majority of the negative details of her relationship with I, and essentially forbade I from talking about it because it would sabotage her efforts with P.  About half a year has passed and B and P's friendship is extremely strained, with B wanting to finally have a heart to heart and explain why he was hurt so badly and see if their friendship can be salvaged.  I think when the truth comes out that P will dump V, and I think there will be a lot of fallout within our friend group.

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I have encouraged her to talk to P and get the ugly truth out first, but I do not think she is going to do this.  She is very, very obsessed with them and I think she will be devastated when it ends.  I want to maintain reasonable boundaries but still be supportive.  Any advice?  Thank you!