How did you find out you had BPD?

A loved one of mine is struggling with their emotional dysregulation, mood swings, and outbursts of behavior at everyday life experiences.  It is to the point where many of their friends don’t want to deal with them anymore.  A therapist diagnosed this person with PTSD two years ago, but the self care for that hasn’t improved their quality of life much.  All their family and friends feel like there is more to the story.  

They’re unsure of who to go to next to get evaluated.  Any help is appreciated!