Imagine closing your eyes and still seeing through your eyelids...
When one is asleep, the brain usually disconnects from the body to prevent the body from physically expressing what you are expressing while dreaming.
"Have you ever had a dream that you were so sure that it was real? What if you were unable to wake from that dream? How would you know the difference between the dream world and the real world?"
Morpheus, The Matrix
To see your physical suroundings in your mind while asleep, the brain recalls from memory the three dimensional physical environment, and virtually places you in that environment. I believe that man is more than just a physical body, that "MAN" is a composite of spirit, soul, and body.
I believe that it is through the human spirit that the minds eye is able to see images just as clearly as it would see images through the physical eye. This is a form of Extra Sensory Perception. I believe that the human spirit sees and records that which is spiritual in nature, and plays back what it has seen in our dreams.
When an individual is sleepwalking, you are seeing their human spirit actively aware of the surrounding environment. And because the mind is semi-conscious, the body is "semi-awake" as if the individual is awake and yet still sleeping; so in their mind they are conscious, but yet do not realize that they are asleep. Such individuals can walk around a room, open the door, and step outside without stumbling into anything; because they are in a heightened state of awareness.
Have you ever had an experience when you were asleep, and you could see and hear things in your dream that was actually happening around you? Like you fall asleep with the television left on, and you are watching a commerical in your dream, and you suddenly wake up and realize that the commercial you saw in your dream is actually playing on TV?
When a person is sleepwalking, they are not actually asleep. They are still "conscious". Have you ever seen someone talking in their sleep, and someone in the room actually says something and that individual responds to them? To the one who is sleeping, they can actually see the individual in their mind.
For some reason, individuals who sleepwalk are not completely in the dream [REM - Rapid Eye Movement] state. They enter into dreamstate, and later partially wake up in their mind (and the brain attains a semi-conscious state to literally reconnect to the body). So you can interact with them as if they are fully awake.
Individuals who sleepwalk should see a sleep specialist, to determine why they can't remain fully asleep and/or fully awaken. The sleep specialist may discover the cause to either be biological, psychological, or physiological. If it is discovered to be a lack or an imbalance of brain chemistry, then some form of sleep medication may help your cousin remain in the deep sleep state that prevents semi-consciousness.