Has anyone experienced something similar with a child or step child with BPD traits that became homeless?
No. But there are still options for her.
I live in a smallish community of 12000 in a rural region. In my community there are 2 large social service agencies that offer supports with housing (homelessness), addictions, mental health and yourh programs up to age 25. Both organizations have trauma informed and dedicated staff. Cities have more options and services and programs.
If It were my sD, I would probably find out what services are available where she lives. If you decide to go ahead with meeting her in public, a hot meal and caring could go a long way. Any conversation she perceives as judgemental of her situation and you may never see her again. Meet her where she is at. During the course of the conversation, you could ask validating questions to determine how informed she is of services where she lives. Cash gifts may bring repeat requests. Hot meals and other forms of kindness might be appreciated. It sounds like supportive services is what she needs.
If she’s not interested, shes not ready. But kind words and genuine acts of kindness can still mean a lot.