British Columbia (Canada) high heels cannot legally be required.
Was the meeting to be in Vancouver, BC, Canada or Vancouver, Washington State USA? The BC Workers Compensation Act prohibits the mandatory wearing of high heels. https://news.gov.bc.ca/releases/2017PREM0047-001151
I also thought the employer could not force women to wear a dress. The women are allowed to wear pants. This was a ruling because a restaurant required the women servers to wear high heels and a black dress. (I have not heard of any discrimination law suits about guys being fired for wearing a dress.)