A college girl is secretly a mermaid.  Unfortunately for her, she is
found out and captured by the (U.S.) government. . .but she is able to
contact a lawyer via cell phone.  What viable legal recourse(s) and/or
laws does she use to regain her freedom?

Assumptions:  (1)  She prefers to avoid publicity (because that could
lead to a lifetime of stalkers); (2) The government does not hide her
away in a totally inaccessable national security zone (e.g., Area 51,
Guantanimo Bay, etc.); (3)  Yeah, I suppose she could ultimately try
to get a presidential pardon or congressional action (as it were). .
.but I'm thinking there are some legal processes and/or hurdles along
the way.

Side note:  I read somewhere that England once had a law which
proclaimed any mermaids found in English waters were automatically the
property of the royal family.  In more recent years, of course, this
law was (allegedly) repealed for being obsolete.  Nevertheless, were
that law still in place and the above scenario played out, I think
Prince Charles would get to decide her fate!  ;-)

Yes, they repealed the law fearing repetition of the incident happened
in the Palace.   Many years ago in Europe, two young princes planned a
dual to settle the ownership question over the Mermaid they got from
the sea.  They could not agree on alternate day ownership!  A scandal
was avoided, it was solved when the king came to know about it ? he
took her.

So it all depends where she applies for protection.  Things would have
been different, if she had come ashore some years ago and had no
aversion to cigars!