>46.2-1060. Illegal sirens, whistles, etc.; unlawful use of warning devices; exceptions.
>
>... It shall further be unlawful for any person at any time to use a horn otherwise than as a reasonable warning...

It was a reasonable warning to him. He was being a danger on the road and it snapped him back to attention, thus resulting in him being alert to what he was actually doing. I didn't do it as a message of "I think you're a bad driver." No, it was to get his attention, which was enough for him to realize that he wasn't aware of what was happening around him.