In quantum mechanics, mixed states are indeed real and are used to describe systems that are not in a pure state. A pure state is a state where the quantum system is in a well-defined state, represented by a single wavefunction.
However, when it comes to macroscopic objects, such as your friend in the other room, the situation becomes more complex due to a process known as decoherence. Decoherence refers to the interaction of a quantum system with its environment, leading to the loss of quantum coherence and the emergence of classical behavior.
Macroscopic objects, including everyday objects like people, are composed of an incredibly large number of particles. The interactions and entanglement between these particles and the environment result in rapid decoherence, effectively destroying any quantum superpositions or mixed states. As a result, macroscopic objects tend to behave classically, following classical laws of physics rather than exhibiting the quantum behavior observed at the microscopic scale.
So, while it is theoretically possible to describe macroscopic objects using mixed states, the rapid decoherence they undergo makes the quantum aspects of their behavior practically negligible on macroscopic scales. This is why we don't observe macroscopic objects in superposition or in distinct mixed states in everyday life.
It's worth noting that the boundary between the macroscopic and microscopic worlds is not precisely defined. The precise conditions and size at which decoherence occurs depend on various factors, such as the nature of the system and its environment. However, in general, macroscopic objects are effectively described using classical physics, while quantum mechanics remains crucial for understanding the behavior of microscopic particles and systems.