If your files are opened on machine A, and the other on machine B, that's possible of course. No offense to the person asking, but sometimes this is often missed.
Same is true if they used the viewer before opening your file, or LT before opening the other people's files.
If they have recently installed the viewer (not sure why, LT2011 can read all files from any AutoCAD version and can handle conversions etc.), uninstalling it would fix the problem too. As you note, Windows controls what gets used, if they can't tell which one they started last this would fix it.