Your attorney is correct, the USCIS regulations require that the employee has a Master's degree, and not that the employer require such degree as part of the job offer, in order to qualify for the advanced degree cap.
Note that such degree must be by a U.S. Institution. The rationale behind the additional advanced degree cap was to stop foreigners who have received advanced degrees in the U.S. from being shut out of the U.S. labor market and having to take their U.S.-obtained skill and knowledge abroad.