My iMac had two 1gig sticks of memory. When Lion installed, I needed 4 gigs.
When I put one 2 gig stick in (meaning I have 3 gigs total now), display works fine. When I add the second 2 gig stick and turn iMac on, computer is on, but display is black. I have interchanged the new memory sticks by themselves in the imac to make sure they are both fine, which they are.
Soooooo......when I have either one of the new 2 gig sticks in (either memory stick--they both work properly), the display works, but when both 2 gig sticks are in, the display is black.
My model (late 2007) indicates computer will take 4 gigs total.
Forgive me but I'm getting a little confused with two computers, having a hard time figuring out which is which right now. BUT....yes the two iMacs both use PC2-5300 DDR2 SDRAM 667 MHz.
The double black screens is more than a little puzzling. Have you reset the SMC on her computer aft changing the ram?