Keep a record of everything and contact Child Protective Services and her obgyn and hospital. They will not allow her to take the baby home, you probably need to speak to an attorney, many have free consultations, or you can find out all you need from cps. You are right, this baby should not be in the same home as her.
Laws vary from state to state but you are probably legally not able to force her to have an adoption. Once a girl is pregnant, medically, she is considered an emancipated minor, and makes the decisions for herself & her baby with no parental consent required (except to abort).
Here is the Human Services Agency info in San Fransisco:
###-###-####
1-800-856-5553
http://www.sfhsa.org/174.htm
Also, she can safely surrender a baby to the state within 3 days of birth without facing prosecution and at no cost. You can do that by taking it to a fire station, or leave the child at the hospital, info here:
http://www.sfhsa.org/228.htm
Also, because she is a minor and creating a harmful environment for the baby, she can be held legally responsible and you may be able to have her forced into rehab. Call the numbers above and they can offer assistance.