Are you perhaps forgetting that this person was involved in organizing the women's march? Do I actually need to find a source for US law suggesting that retaliation against women reporting sexism is in fact illegal?

This is basic compliance training for any US employment, and the EU has similar laws. Where do you live and work that you don't know this?

Organizing the women's march is like reporting sexism? I don't think so.

Women can organize to protest sexist conditions. That's a protected activity in the US.