As I understand it, the basic idea is that the passive voice implies that the subject suffers some action upon himself, whereas the adjective just describes the state of the subject.
So: The water he drank was boiled. <--- boiled is an adjective.
The water he drank had been boiled <--- passive voice.
During the experiment, the water was boiled on the Bunsen burner <--- passive voice.
In your case, it's a very close call, isn't it? Because you told him something though, and that's what flabbergasted him, I'd say it's a passive. But does it matter, in the end? The meaning is the same. If someone insists it's an adjective, I won't protest.