Turning on Dishwasher With 4-Hour Delay During Extreme Cold Weather
Turning the dishwasher on it shouldn't make a difference as long as they leave their kitchen tap run a little stream of water.  This should keep the main service from freezing.  Inside the house as long as you have a heater (which you said setting the thermostat to 70F) which is what I keep my house around at all winter.  They will not freeze inside the house.
It has been -40 here for a couple weeks now.  As long as you have a slow stream of water the lines should not freeze.  I do live in Canada and most of our water and services lines are 8 feet or deeper outside however.  I cannot attest to how or where your water and sewer services are ran inside or outside of your home and how much insulation is in your houses.
We have had to run garden hoses across lawns in the middle of winter to supply houses with water when their main services get frozen.  As long as the water flows it should not freeze.  DO NOT turn it off however.
Depending on the depth of your sewer lines and where they run you could possibly freeze them up.  (Like I said ours are usually deep in the ground when they leave the basement of the house).  A larger stream of water would prevent this but would use more water.