Sounds like quite a party and they can go to trial. Hope the judge does not think they are dipsh--s if found guilty
My name is Stephen R. Cohen and have practiced since 1974. I practice in Los Angeles and Orange County, CA. These answers do not create an attorney client relationship. My answers may offend I believe in telling the truth, I use common sense as well as the law. Other state's laws may differ.. There are a lot of really good attorneys on this site, I will do limited appearances which are preparation of court documents it is , less expensive. However generally I believe an attorney is better than none.
As already noted, your friend can plead not guilty and take the case to trial. The question will be whether the public display of the three-story beer bong was otherwise disorderly conduct that tended to cause or provoke a disturbance under the circumstances. The police officer's unprofessional conduct doesn't really go to either element, but may be the cause for a complaint regardless of the outcome on the citation. All police departments in WI are required to have a citizen complaint process in place. Most require an in person appearance at the PD to lodge a complaint.
This answer is provided for general information only. No legal advice can be given without a consult as to the specifics of the case.
Of course you can fight the charge. Whether you win or not is another matter. Based on what you describe, a number of defenses might be possible.
While a Class B misdemeanor is a less serious crime (it's not like a murder, for example), it is still a crime and therefore serious. To protect his future, your roommate should hire an experienced defense attorney.
My answer to your question is based on the facts that you provide in your question. Additional factual details about your situation could change my answer completely. The law in inherently uncertain and always subject to change.