This is great advice, and so is the suggestion about BAT, which is basically the same principle - you work under your dog's threshold so that she doesn't respond with her normal reaction. The trainer you spoke to might not have been that far off the mark in one sense, although his advice certainly was stupid. However, we often see dogs that are highly reactive when they are on leash, behind a fence, in a crate, or anywhere they feel trapped, but when they are off leash they do well with other dogs. That, in fact, is really the main difference between reactivity and true aggression. The aggressive dog behaves aggressively both on and off leash, while the reactive dog only reacts while on leash or when the personal space gets invaded.