Yes, the way I wrote the scenario makes it seem like he deliberately got himself into an awkward situation for little benefit in return. And I see how this weakens the scenario as an illustration of the problem. So let me try improving the scenario:

Imagine he determined that refraining from disclosing the information to his mother was ethical. A week later, he finds himself in a similar situation. He wants to drink a couple of beers, but knows that by the time he'll finish, he'll need to drive his mother. This time he has no qualms about drinking, making the beer-drinking pleasure worth the consequences.