What? EAM asking her to wear makeup (which as far as I know they didn't) or him complaining about it? I can't imagine any possible scenario under which the latter would be illegal what could he be accused of?
As for the first well some Googling suggests that employers can indeed insist on female employees wearing makeup if it is 'appropriate' The Devil in the Detail is figuring out what's appropriate.
Dress codes are allowed to take account of gender and don't have to be identical providing they are of 'equal' weight (another subjective word there)
EAM's dress code is/was smart casual at the office and business attire if meeting customers. There are some guidelines, male employees were required to wear a suit and tie. Female employees were banned from too short skirts and too high heels. I'm freelance contractor scum rather than staff so I don't have my copy of the handbook anymore but it really wouldn't surprise me if makeup for female employees was mandatory for meeting customers and optional for the office. This happened during lockdown so she (like me) was working from home.
One amusing thing I do remember from the handbook was that it did actually say skirts were optional for women but banned for men so I've always wondered if someone had ever tried to push the limits.
I don't know what if anything her manager said to her but she was re-assigned to another project within a week (which is how I found out when I emailed her to ask something) and replaced with a man.
This must have added a couple of weeks to the project which was costing £tens of thousands per day whilst he caught up with where she was.
There were four client managers on the project (though to be honest you could have replaced all but one with cardboard cutouts for the value they added) and all four of them stayed involved with the project.
I don't know which particular one complained but there was one that was a particularly stupid and annoying twat so I mentally accused him but I don't know for sure.