Its not sounding good. How long have they had this machine? Under what circumstances did the video stop working?
If you've replaced teh whole motherboard, and still no go, all you've got left is the PSU and the CPU. I would try a diff PSU just for shits and giggles.
In my experience, a dead CPU will exhibit the same symptoms you're experiencing, but they very rarely just up and die on their own.
Also make sure you unplug everything non essential to operation, cdroms, hard drives, pci cards, just run the motherboard, cpu, ram, and psu. If it still doesn't post, you know its one of those 4 things, and since you've already swapped out the ram and motherboard, you can now limit it to the remaining two items.