Is a person's saying, as a joke and without intention of divorce, "At that point, I would have divorced their mothers," considered a divorce?

A divorce uttered in jest is valid. However, the man's statement was not a divorce but a promise of it. A promise of divorce does not constitute a divorce, nor is one obligated to fulfill it. Therefore, your friend's divorce did not occur through what he said. He should be advised to avoid jesting about such matters and to be truthful in his speech, even when joking, for the Messenger of Allah, peace and blessings be upon him, said: "I guarantee a house in the middle of Paradise for one who forsakes lying, even if he is jesting."