Oh, ok. In that case, I'll try to break the problem down a bit. What does "separate universes" mean, exactly? Your friend seems to be saying that people leave "your universe" when you can't see them, but in what sense are they actually leaving your plane of existence? Also, what is different about "your" universe versus someone else's? And how is it that two or more universes can "communicate" and overlap when the people in them can see each other? And does it have to be eyesight? What about talking to someone on the phone?

For these reasons and many others that I neglected to list, I don't think the idea that everyone occupies their own universe is at all coherent. It's grammatically correct, but the concept just doesn't make sense.