Her son claims the booze was left by a passenger. That's the oldest dodge in the book.
But even if it were true I'd still hold my kid responsible for allowing alcohol, regardless whose it is, in the car. Not good.
I probably wouldn't have sold the car. That seems kind of harsh. But even if you decide to punish the kid by selling the car, then do so and let it go. But don't humiliate them publically by taking out an ad. Overboard IMO.
My daughter just got her license, and she knows that her parents have certain rules that better not be violated or else.