This is a bug we've discussed before. It seems assoviated with particular actions in certain projects and can be duplicated by others.
Somehow a phantom marquee block gets associated with the camera position.
I suggest that your "friend" merge the file into a new project.
If anyone wants to see this in action, PM me and I will dropload the animation I made with my own project. As you rotate around the structure a block of invisibility cuts. haha.
Dwight Atkinson