Scalding hot water poured onto each dandelion. It even kills the long taproot. It will kill whatever grass is in that spot, but you can re-seed and it grows right back (the grass, not the dandelion)
You can also run around with scissors and just cut the heads off every one that starts growing, and toss them in the trash. If the bloom isn't there, they won't go to seed and spread.
Keep your lawn grass at a height of 2 - 2.5 inches, because shorter height than that can kill the grass which allows a dandelion to germinate more easily. Corn gluten meal on the lawn will stop the dandelions from germinating/growing also, but it won't kill the existing plants. Do NOT use fertilizers that contain potassium! It increases dandelion growth and spread approx 20x.
Also .. if you garden - you may want to convinve him to just leave them there. The roots aerate the ground, and the plants themselves extremely attractive to ladybugs, which are notorious for eating up aphids. They also suck up a good amount of ground minerals, which are returned to the soil when they die.