It's a hardware issue then.
You've ruled out the RAM so the next step would be the power supply.
You'll have to exchange her PSU with a good compatible one and give it a try.
You can also check for any over heating issues. Check for proper fan operation on the CPU and GPU if there is one. Check for dust accumulation, especially the CPU heatsink.
If all checks out, then it's on to the GPU. If the GPU is built into the motherboard, then there is really nothing you can do. If there is a separate GPU card, change it for a working one.
Next would be the hard drive. There are diagnostic tools available for each hard drive manufacturer so download them, burn the image to CD, boot to the resulting CD, and test the drive.
Eventually you will find the culprit.