This is a potential lawsuit waiting to happen. You can not leave a reactive dog with just anyone. That person does not know your dog's unique triggers and signs of distress. Your dog does not know that human and does not feel safe with them, and as a result will likely only act worse and will not listen to their commands. There also does not sound like there has been any attempt at meaningful training to help with the reactivity. This is a recipe for disaster. That poor dog.