Charge her too
Not knowing the real age of the victim is no defense. Really? Believing fraudulent statements given is not a defense? She needs to be charged with fraud. She presented herself in one light, when she was not even close to it. Her statements may be protected speech by 1st Amendment, but her intent and presentation was fraudulent and illegal in doing so.
A minor who is served alcohol can get the server and establishment in trouble because they believe the ID presented should it show 21 or over. The minor can be charged with Minor in Consumption and/or Possession (two charges there). Both parties can get into trouble in the end.
The girl should be charged too since the guy is also.