The maximum penalty on Public Intoxication (assuming that it's not your 3rd or more) is a $500 fine, plus the costs of court. If it IS your 3rd or more, more significant penalties may be involved.
The officers are not required to give you field sobriety tests or a breath test when arresting you for public intoxication. However, if you are convicted and it is a first or 2nd PI, then you will only be fined. If it is a 3rd or more, you could get jail time.
Hire a lawyer and fight it. The burden on is on the prosecution to prove it and they don't have much proof based on what you report.
(He arrested to you to "mess you up" as a witness for your friend. He wants the State to be able to argue that you were intoxictated therefore you are not in a good position to testify that she was not intoxictated.)