What can be done if I have a 20 year old niece who met a guy who has been prostituting and she has been arrested twice?

Unfortunately, she will have to go to court in both states. Often in CA they will reduce the charge to disturbing the peace and require an AIDS test. I cannot tell you what happens in NV.

PC 647(b) Every person who commits any of the following acts is guilty of disorderly conduct, a misdemeanor: (a) Who solicits anyone to engage in or who engages in lewd or dissolute conduct in any public place or in any place open to the public or exposed to public view. (b) Who solicits or who agrees to engage in or who engages in any act of prostitution. A person agrees to engage in an act of prostitution when, with specific intent to so engage, he or she manifests an acceptance of an offer or solicitation to so engage, regardless of whether the offer or solicitation was made by a person who also possessed the specific intent to engage in prostitution. No agreement to engage in an act of prostitution shall constitute a violation of this subdivision unless some act, in addition to the agreement, is done within this state in furtherance of the commission of an act of prostitution by the person agreeing to engage in that act. As used in this subdivision, "prostitution" includes any lewd act between persons for money or other consideration. Penalties for Prostitution in California Misdemeanor penalties Prostitution, solicitation, and agreeing to engage in an act of prostitution under Penal Code 647(b) PC are all misdemeanor offenses. If convicted, you may face: up to six (6) months in county jail, and/or up to one thousand dollars ($1,000) in fines. Prostitution / solicitation are a "priorable" offense..., which means that the punishment necessarily increases with each subsequent offense. If convicted for your second prostitution or solicitation offense, the judge will sentence you to a minimum of forty-five (45) days in a county jail. In addition, if you are convicted of a third prostitution or solicitation offense, the judge must order a minimum of ninety (90) days in a county jail. In addition to these penalties, if you committed the offense (1) while using a car, and (2) within 1,000 feet of a residence, the court may suspend your driver's license for up to thirty (30) days, or issue you a restricted license for up to six (6) months. If it can be proven that, your niece did not have specific intent to so engage in prostitution or that she did not manifests an acceptance of an offer or solicitation to so engage in prostitution then she cannot be convicted.

Petition the court to become her Guardian.