MIL's age? Eyesight, good, average, poor? Dexterity? Any arthritis?
I will recommend a Mac 99% of the time especially for the older crowd.
However that is a smaller profile box. Smaller screen, smaller keyboard and I find the elders don't take well to trackpads.
Does it have to be a laptop?
http://shrevesystems.com/ have on the lower left of the page an Intel chipped Mini at $350.
Then go and order a 22 in flat panel from newegg, tiger. You can get Envision or Niko's (same OEM) for ~$250. You are at $600, plenty of budget left for peripherals like printer and BlueTooth keyboard etc....