My daughter is a heroin addict. We live in NJ. She is 28 years old but acts 14. She has legal problems.

If she signs a power of attorney, she can always revoke the power of attorney.  A guardianship is an option, but it would require two doctors to certify that she is legally incapacitated, meaning she is unable to govern herself and her own affairs.  If you became her court appointed guardian, you could force her to remain in rehab, she would not be able to sign herself out.  You need to consider how you would pay for the rehab, if she does not have insurance, and what happens if she finishes the rehab.  You can always "reverse" a guardianship - you can go back to court and have a doctor state there was a return to capacity and she can have her autonomy back.  Good luck.