Black band on upper third & dark gray band on middle third of display

iMac G5 Intel Early 2008 24" A1225 EMC2211

My 90 year old mother loves her iMac. The display has gone black in the upper 1/3rd of the screen, then the middle 1/3rd of the screen is a dark gray. This dark space does not flicker, and is getting worse; soon the entire display will be covered in dark and unreadable. We took it to Apple's Genius Bar and the Genius said they won't fix it due to age. I'm trying to figure out if I have to replace the display, or if it's a backlight issue, or if it's something else. If we shine a flashlight on it, all the icons are there and all the programs such as Mail work. Of course Apple tried to sell her a new computer, but she's on a very fixed income so I'm trying to fix it first. She can't afford a new computer even though she looked at the Mac Minis, and it's her baby, so I'm really hoping to fix it.

I've tested it with an external display and it looks fine on an external display. The iMac display is getting worse though.