Your menacing presence at the door was overkill and probably unnecessary. You should have remained hidden in the background and let her handle things. You could still have intervened if required, but it doesn't sound like that would have been necessary.
Whether or not he was naive to "yammer on" in your presence, obviously either he was early, or she fucked up in her booking process. If he was early, she should have told him to come back at the correct time. If she had fucked up, she should at least have apologized and told him to get back to her later. You seem to not have considered the possibility that it was her fault. Of course, we'll never know for sure.
If the situation had been reversed, you probably would have been very pissed off. Given your apparent taste for confrontation first, would you have tried to use your size advantage to threaten a guy who was already there?