In general, "it's unlawful to harass a person because of that person's sex," according to the EEOC page on sexual harassment.
Harassment can include:
Harassment is also considered illegal when it creates a "hostile or offensive work environment" or when it results in an employee being demoted or fired. The harasser can be of any gender, and the victim can be of the same or different gender. The harasser can also be a coworker, supervisor or manager, client or customer.
Since she called out her harassers, the client's company has called their behavior "inexcusable," but she has yet to receive a genuine apology. She also said her employer supports her and will not work with the company on the call.
With her realization that sexual harassment is defined by a federal regulation like the EEOC, she feels it was within her rights to call out the men on the Zoom call. In fact, harassment of any kind must be acknowledged and talked about in the workplace. Employers must set a zero-tolerance policy on sexual harassment that includes what's defined by the EEOC and provide safe procedures for preventing, reporting, and addressing those that are causing someone else to feel uncomfortable.
Pre-employment and periodic background checks on employees are suggested to determine if the harasser has previous complaints against them. There may be other signs as well, such as a manager with an unusually high turnover on their team. Behavioral interviews and other references may also reveal unwelcome conduct. But most importantly, when someone claims they've been harassed, believe them.