A child has an imaginary friend...

This keeps on bugging me. It's not my child, but he is our neighbor. The story goes like this... Of course, I'm not aware of his conditions. One morning when our door was open, he suddenly walked inside and keeps on visiting each bedroom and corners of our house. He's starting to murmur to somebody which we don't actually see. He seems that he has his own world. Is that something normal? I'm reluctant to ask his mother (our neighbor) about it.