I suspect if he's running Tiger he's well out of warranty (just based on timeframes).
OP, if it doesn't show up in a screenshot, then it's definitely a hardware problem. If you can connect your computer to an external monitor, you can help determine if it's graphics card or display, but usually anything related to the graphics card will shift in color or position as you use the computer. It's likely the display panel or the display cable (internally). You'll need to get someone to fix it for you, or decide to live with it (it will likely be expensive). For reference, iResQ will replace just the panel for
$279 plus shipping. That price could go up if the cable is needed too.
jW