My two cents!
Get your MIL to measure the cast from the heel around the instep--use that measurement to determine the # of cast-on stitches. Unless you know already, you might have to knit a wee swatch to see what gauge you get with the yarn and needles you're using. Then, multiply the stitches per inch by the number of inches around the heel/insteap, and voila, you have your number of cast-on stitches!
I also recommend the spiral rib socks; they'll fit over a cast much better than an actual sock will; the angle of a sock with an actual heel might make the sock difficult to get on and off.