Know anything about Northern Virginia?

My friend Liz (names changed to protect the innocent!) is A-Mazing. I recently learned just how amazing. She and her younger brother are both adopted, different birth families, and while she had the benefit of a healthy time in her birth mother's womb, her brother did not. He has all sorts of mental health and developmental disability issues, compounded by a stay at one of the well-known but has-abusive-tendencies residential treatment centers.

Liz's parents aren't able to provide her brother with the support that he needs, so Liz is his primary support. She is his representative payee for his SSDI/SSI and pays out of her own pocket for him to see his current therapist.

The problem is housing. Liz's brother lives in Northern Virginia, and needs to stay there because that is where his therapist is. (Among other reasons, but this therapist has been successful at getting Bro to take his meds.) Liz found Bro a room in a house that he could afford, but it turned out that the roommates were dealing drugs, and Bro didn't want to stay there. So he moved out and is now homeless.

I want to help Liz help her brother, and am wondering what resources might be available for a mid-20s adult adopted as an infant with mental health and developmental disability diagnoses in Northern Virginia.

Thanks for any help you can give!