I made that post after actually researching Wisconsin law on the matter. Recording a conversation without at least one party's consent, especially with the use of amplification in order to make the recording, is considered wiretapping/eavesdropping, which is punishable with a minimum fine of $10,000, and possible jail time. A judge may well find that there was no expectation of privacy, but I think the driver could make a case otherwise, especially with the method by which the recording was made. It's all good if you disagree.
And yes, he would have to have suffered damages to have grounds for the suit.