I have provided the Texas Atty General's explanation for you to provide the officer as to why it is illegal to detain and return a 17 yr old. I also provided the harboring statute in the second link.
An unemancipated (2) seventeen-year-old is not a "child" within the Juvenile Justice Code.
https://www.oag.state.tx.us/opinions...htm/ga0125.htm
Sec. 25.06. HARBORING RUNAWAY CHILD.
http://www.statutes.legis.state.tx.u.../htm/PE.25.htm

