(I've posted in diff ways about this situation a bunch of times before but I'll just start from the beginning in explaining it).

I have BPD and so does a friend I was very close with for several years. We lived together in a share house with a bunch of other people and were super, super close. When I first moved in, we didn't know each other at all and got to be close through living together. She's very emotionally turbulent, she doesn't have consistent relationships and they're rarely healthy. She loses a lot of friends. During the time I lived with her, a lot of close people left her life. We never had conflict when we lived together, I think because I understood the rapid shifts in her moods a lot better than others, but also because I find it hard to have conflict. 

At the start of last year, I met someone from my work who I dated for a few months and then we became just friends. I introduced this person to my housemate and we were all friends.

I moved back to my hometown due to covid but the three of us kept in contact in a group chat talking all the time.

Then a month or two ago, my friend/housemate (E) called me to say that the two of them (her and my ex from work) had been sleeping together for about a month and hiding it from me. I pretty much just explained how hurtful that is and that I don't want to be friends and that was that.

We had a bit of contact the other week and E was pretty much saying that my relationship with this person had been ages ago and it should be fine and she can't bear to lose me as a friend, and that the fact that the two of them have feelings for one another doesn't invalidate how much she cares about me. I was really upset and overwhelmed and said I'd speak to her later instead.

The thing is, I feel so much better without E. I was the most unstable I had ever been last year when we lived together, I think her BPD exacerbated mine a lot. She has really tumultuous relationships and always is engaged in some sort of drama, and is so enigmatic she always sucks other people in. I find that not being around her, because we'd been so close for so long, I'm finally seeing that she'd normalised a way of life for me that is really unhealthy. I'd been used to constant turbulence because I guess she'd demonstrated that for so long around me (I should also add here that she's a fair bit older than me). Both her and the person she's now dating engaged in so much drug use and reckless behaviour and I think being around that was awful for me. I'm not saying they are entirely to blame for my state last year, but it feels like it didn't help at all.

I was telling all of this to a friend and she thinks E needs to know this. But I think it could just hurt her? However I also don't want to be disingenuous. A lot of the people who have stopped being friends with her in the past haven't said these things out of not wanting to hurt her. The main thing I want to convey is that she needs to get better and really commit. The whole time I've known her she'll commit to therapy for a few months maybe and then always go off the deep end and quit, then a few months after that pick herself back up. I can't be around that. But is it my place? I feel like if I speak to her about this she'll split and either become very suicidal, or very angry at me.