The guy's FOS.
Alcohol based mouthwash (Such as Listerine) will set off a breath machine, but only within a few minutes of using it.
By the time he was finished with the normal parts of the traffic stop, the field sobriety tests, and the car ride to the station for the breathalizer/datamaster the mouthwash would have long since worn off. Most departments require at least 20 minutes of continuous observation before administering a breath test - specifically to preclude the mouthwash defence.
The small roadside test we use - the "Alcosensor" in my area - is not admissable in court. It is only a pass/fail test. It gives a pretty accurate BAC level, but it's accuracy has not been properly certified in court. They only recognize it as a positive indicator of alcohol - regardless of the actual reading. We will generally use it as the last field sobriety test before arresting someone and then taking them in for a breath test. My department only uses blood unless we are dealing with minors under 18. Then we use a neighboring departments datamaster (an advanced breathalizer).
You can bail the guy out if you want, but he's feeding you a line of feces.
-K