Alright, back into this.
Person A unlocked Stanley's room through a guest room key that was either stolen or obtained through other means. Person A and the killer, Person B, both entered Stanley's room. After Person B killed Stanley (Person A simply witnessed this), both of them left the room, and Person A relocked the door from the outside.
Slight adjustment.
Stanley called Simon's Office phone to invite a person in. Instead of Simon answering, a person (named Person A or Person B) answered, and requested that they figure out a way to meet up in his guest room. Person A unlocked Stanley's room through a guest room key that was either stolen or obtained through other means. Person A and the killer, Person B, both entered Stanley's room. After Person B killed Stanley (Person A simply witnessed this), both of them left the room, and Person A relocked the door from the outside.