I wanted to add...I had severe migraines for 25 years. I never took narcotics for them. If she hasn't seen a neurologist, that might be a start. The narcotics cause rebound headaches, so now there isn't really any way to separate the causes from the narcotics. It's possible the daughter has inherited that tendency, migraines among female family members are common. Narcotics are not the solution, there are specific non-addictive pharmaceuticals for migraine. I spent a lot of time at various doctors, hospitals, and psyche specialists before finding the treatment that worked. So this "headache" excuse is just an excuse.
I agree with Thomas that inpatient care is the answer, but you can't make that happen. Keep working on the custody thing. My heart breaks for both those kids - what an awful life. They aren't throw-away people.