Yeah, I know this post is old. Disclaimer: I am not a professional repairman. In a past life, I did sell appliance parts - over the phone!
Check the "obvious" stuff first... dishwasher drain hose not kinked or blocked. If the dishwasher drains into the kitchen sink drain, make sure that drain is clear. If it drains into the Disposall, does the tenant know to run the Disposall to clear it out before starting the dishwasher?
Like Colleen said, make sure they've got the "heated dry" switch or cycle selected.
Sometimes you get lucky and the diagnosis is easy, like they've dropped a pot on the heating element and it's bent like a pretzel or broken in two. In that case, a new heating element will probably fix it. Sometimes you have to go digging, though.
If you decide it really is busted, it might pay to get a used, working dishwasher, swap it out so the tenant is happy, and then fix the busted one at your leisure.
The way most older and less expensive dishwashers work is...
All the power runs through the door latch, so nothing works unless the door is closed and latched.
The timer turns on the water valve to let water in.
A float switch turns off the water valve once enough water has been let in.
The timer starts the main motor, which pumps water through the sprayer arm(s). They move around by the force of the water, like a lawn sprinkler - they don't have their own motors.
After a while, the timer stops the main motor, turns on the drain valve, and then turns the main motor on again.
The main motor then pumps water out to the drain.
After a minute or so, the timer stops the main motor, turns off the drain valve, and turns on the water valve again. Repeat from 1 to 3 times or so.
At some point in all this, the timer also turns on the release for the main detergent cup, to let detergent in.
After the last pump-out cycle, the timer turns on the heating element in the bottom of the dishwasher, and leaves it on for an hour, plus or minus.
There isn't a thermostat that turns the heating element off and on (like an oven). There will be a thermal fuse or high-limit thermostat that turns the heating element off if something has gone badly wrong and it gets really hot inside there.
When the timer is done, it shuts off the heating element, and hopefully the dishes are clean and dry.
The end.
Some newer dishwashers have separate "main" and "pump out" motors. Some fancy ones don't have a heating element... they have a blower that attempts to air-dry the dishes.
There is probably a wiring diagram and some service information on a small sheet of paper folded up underneath the dishwasher - maybe in a little envelope. You'll have to take the kick panel off to find it.
If the problem is electrical, it will help a lot if you know how to drive a multimeter (digital multimeter, volt-ohm-meter).
I hope this helps!