When someone enters a licensed residential rehab center in California, the facility assumes a duty of care.
California regulations require staff to conduct face-to-face checks on patients at regular intervals.
The facility doesn’t just have a legal obligation; in most cases they’ve explicitly marketed themselves as providing 24/7 supervision.
Negligent supervision is what happens when that promise breaks down.
If the facility had a duty, they breached it, and that breach caused the death, they may be held liable.