Whenever someone has a high electric bill, you can do an energy audit. Either the utility company will do it for you, or they can offer instructions on how to perform one.
It's easy to do. I do energy audits regularly and it really uncovers the problem.
Just an example: Years ago I was leasing a house and had very high electric bills. We found that the 220 wiring was shorting the 110 in the breaker box. The box was improperly wired.
It would be a good idea to have a pro do the audit if you don't find an energy-hog appliance in the renter's unit.
Also, high electric bills may indicate a potentially dangerous situation that could result in a fire.