The argument was made in a letter to City Attorney Jan Goldsmith. Filner's lawyer Harvey Berger said the training was scheduled but the city trainer canceled and did not reschedule.
Filner may not be facing a lawsuit today if he had undergone the classes, Berger wrote.
The City Council has already unanimously voted to deny Filner any taxpayer money for the case. The council has also voted unanimously to sue Filner to require him to pay for any damages or attorney fees out of his own pocket if the city is held liable.
Eight women have come forward, alleging Filner made unwanted sexual advances. Seven of the nine council members are urging Filner to resign.
But Filner, who is 70 and divorced, is refusing to step down. Instead, he plans to enter two weeks of "intensive" therapy Aug. 5.
The Associated Press contributed to this report.