What defense should I use for an underage drinking charge issued because of an incorrect breathalyzer result?

As far as the citation, you are not signing an acknowledgment of guilt, just like they say on cops, just that you got a copy of the citation. His test showed a reading so he gave you the test based on the behavior of your friend which a reasonable person could then suspect you of the same behavior based on young men all getting drunk and go to a concert together. It happens all the time. So, the only thing you can do is get a lawyer and let them tell you what the options are. The only thing I see from a non legal perspective would be the device needs to be tested to see if it was off and it may be too late for that. If you are going to fight this, you need a trained and experienced attorney that knows the ins and outs of breathalyzer defenses in your area. If you are going to try to do this yourself, and there was not a cleaning of the device after your friends tests, but before yours, then there could have been something left over. You might also google the device and see what the margin of error is and what sort of problems the device has.