She does not have a right to decide where she wants to go at this point because she is only 15. That being said, her dad has a legal duty to continue to support her and could be charged with neglect of a dependent under IC 35-46-1-4.
If you are truly seeking to do this the correct way, then you should consult an attorney about seeking guardianship of your son's girlfriend. This way you would have the legal means to sign appropriate medical documents for her and act as a custodial parent. If she just moves in with you, you may find it difficult to act as her guardian or custodian without a court order.
Answered on Aug 14th, 2012 at 8:45 AM