Yeah it would take a 1.5HP pump running 24/7 (no cycling on/off) to use that much energy. I would turn the power off to the pump for a night and see if the daughters apartment goes black.
Funny, I just saw this and it's what I wrote earlier - guess we are on the same page. But on re- reading the guys situation, the breaker for the pump is in his house, and I suppose the meter also. So the landlord would have to be quite a devious genius to be stealing power for the new apartment. Now, if the meter is outside on a pole somewhere, there are many simple ways for him to tap off your juice and send it to the water heater of the new, illegal apartment. If you are not electrically proficient, it will be hard for you to track any power theft. Might have to hire an electrician.
Again, if you are using electric heat, all bets are off on winter bills. You will go sky high. The best way to break a lease is to not pay the rent and send a certified letter with a list of the defects in the house. Move out, and believe me, he won't chase you for the lost rent because its too expensive. Finally, tell him your first stop after he tries to collect any rent due on the lease is the county building department, which would, in this county, cost him from 3 to 15,000$, depending on if he exceeded the septic capacity.