Maybe it's just a cultural thing, but her outfit that morning did and does not qualify as business casual here. Appropriate for a nightclub, yes, but for the office? No. It's hard to tell now with so many people working from home, but in the past most women wore pants and a blouse.
I think pants and shirt are generally business casual for
both all genders here, too. But women tend to wear skirts in a casual way, especially in summer, because they're cooler than long pants and more attractive (and suitable for the office) than shorts. But I hear you -- leather skirts would be at sort of a different level of dressiness. Maybe she had plans after work?
My current work-at-home "business casual" is just a step above my "loungewear," which is just a step above "pajamas." Which category any given garment is assigned is usually a matter of how many holes it has.
Jeans are verboten. However, here's the thing. Certainly my employer's policy is "no jeans," but what that really means is "no blue denim." For decades, even long before I came to my current job, my trousers of choice for work from about October to April have been corduroy pants from Lands End. These trousers are made exactly like jeans, rivets, pockets, and all (Lands End has even described them as jeans, IIRC), but because they're corduroy and not blue denim, nobody notices.
Yeah, I think it's more the fabric than the design mainly because of the history and cultural symbolism of denim, especially once it gets faded or has holes in the knees. Or when you purchase it with pre-made fading and holes at prices higher than intact jeans would cost.