I have CPTSD which I know is a lot of overlap but this is about someone else. Someone I knew told me they were in DBT for Bipolar II, but a therapist I talked to said lots of people are misdiagnosed and when I told them about our platonic relationship exploding they said it sounds like BPD. 

The biggest thing that points out to me is, they told me how they were sexually assaulted, in relationships with abusive addicts, and an alchoholic and heroin user themselves. I was crying a lot for what they were going through. 

There was this incident I was really creeped out by. When I went over to their house because I thought they were suicidal (they were), she denied it at first but then started crying about how she looked up stuff online about how to do it. She has suicidal ideation issues (as do I). She apologized and wiped her eyes and I said it was fine, at least she's honest with her emotions. I took a deep breath and said "Cause I really feel bad about what happened to you" and tears started to well up in my eyes and my voice broke. 

She immediately got cold, her teary eyes went away and she said (I felt, judgementally) "youre really empathetic huh". I just put my head back and said yeah, and she changed the topic onto other things. 

They're also a person who is very deadpan. When I asked to borrow a book she held it to her chest and had such an exaggerated expression of mock-grief on her face, to the point where I took a step back because her face was so distorted. Then it went away as quickly as it came and she gave it to me. Again I was very weirded out because they told me they weren't BPD but Bipolar. 