Those with Borderline Personality Disorder struggle with healthy boundary setting. From my perspective this is part of their black/white, all/nothing thinking. Sometimes it's obvious when someone has poor boundaries. The story I always tell to describe what poor, or low boundaries look like is this personal one from when I was 15 years old. I had just walked off the city bus to walk to go home, as did another classmate I had not seen before. It was the first week in school, so I guessed she might be new. I began talking to her, figuring if nothing else it would give me someone to walk home from school with. Our walk and only conversation lasted 8 minutes and in it she told me her mother had killed herself, she was now living with her father, he didn't know how to cook, was "icked" out buying her tampons, she had an array of mental health issues, told me what medications she was on and showed me scars from when she tried killing herself a few months back. My radar went into the red danger zone and I immediately knew this was not someone I would want to be friends with, even if it was 16 minutes a day round trip.
Those with all/nothing thinking struggle with making, having and often keeping friends for this primary reason; "they are either my all-time best friend or my worst enemy." This was a quote from a client, whom I didn't at the time know much about BPD to diagnosis. I have my clients create boundary gardens to illustrate this point and have a visual for where their friends lay. Below is the outline of the garden I have them fill in.
It doesn't solve their boundary issues, but it gives them a starting point and a nice visual to begin working on.