Is a parent obligated to pay for the cost of an 18 y.o. to live on their own if in hs?:
Depends on your state...Here in Nebraska Legal Majority age is 19. Most states it is 18, but there are special circumstances in most custody agreements that support is necessary through high school. Sounds to me like your daughter needs a swift kick in the pants! It is a very difficult thing to deal with...watching your daughter self-destruct but it sounds like you have no choice. IF she was still living with her father you should be giving him money towards her support...if she is "on her own" in her own apartment, you have no obligation to pay an 18yr olds rent. 
  I would tell her she is on her own...IF she wants to come home and finish High School she can...but if she wants to be a Junkie...she is on her own. She can move into the Dope mans house, get a job, or join the army. All you can do is keep a light on and pray...but you dont have to pay...