No one who has lived has ever seen it. So it could become visible when let in. No where does she say others have seen the creature impersonating their loved one/friends. Either they never live to tell, or they are afraid to. It’s unlikely that having her friend paralyzed on the floor is unique. If it has happened before, it’d be part of the legends (if anyone lives to tell the tale). So…..either Max is gone when she goes back or….she’ll be next and both will be gone before her parents return. IMHO, there’s a very good chance there’s more than one.