That makes it more difficult, at least from the standpoint of fighting the charges.
Had she been arrested, and had her confession been made during or after the arrest, it is possible you could have challenged the admissibility of the confession. Under current law, confessions made during arrest or subsequent to arrest must be "warned" (the police must give a warning that the statements can be used against the individual...the so called Miranda warnings)
What you describe sounds like a clear case. She had been drinking. She was asked and admitted.
So you she can certainly "fight" this...but the ability to win I think will be difficult since I do not see a way to keep her confession out. And if the confession is admissible, that is enough to sustain a conviction
She can certainly demand her day in court
...it is possible the cop does not show up and the judge dismisses the charge. But if the cop shows up, I suspect her ability to win this is not good.
That said, the punishment for this regardless, even if she pleas not guilty will likely be nominal, a fine and perhaps a requirement she attend a substance abuse class
Let me know if you have more questions...happy to assist if I can