Flying player

Good morning, I'm Smooth_UnicornYT and this is the bug in question: I'm building in my world (if you want to visit it, it's called school-zen) and at a certain point I notice something strange. I call my friends and I get closer and I discover that it is a player who is flying without being in creative. Upon checking I discovered that he wasn't even playing but that he had come earlier. Since I don't have an account, I can't know exactly what happened. Thank you