I was born in Knoxville, Tennessee, and I have never in my entire life heard "y'all" or "you all" used to address a single individual--- outside of movies or tv shows, that is.

In fact, I've had discussions on this topic with several people from Georgia, Mississippi and the Carolinas, and we all cringe when we hear "y'all" used as singular. I suppose that a rogue, singular "y'all" may exist somewhere in the south, but is far from mainstream.

As for the original post, I don't know whether "Hi all" is unique to the American South, but in my experience, it would be interchangeable with "Hi everybody." in casual conversation.

Am I the only one here who can't help but wonder if the original poster's ethnicity has some part to play in the advice she was given? I know of many people (myself included, I must admit) who would raise eyebrows at a German walking around saying "Hi all"---especially if it were accompanied with a wave.