I see a mum on the school mum every morning wearing very see through leggings. Today, she bent down in front on me to get something out of the pram basket and I was greeted by the view of her arse through her leggings. She had a pink thong on under thin black leggings and a top which stopped at her hips. So everything was visible, I felt embarrassed on her behalf. A longer top would have covered a multitude of areas.
So my aibu is if I see her again should I tell her that her that her arse is on view? Diplomatically of course. I'd want to know I think. It's hard to see how see through a fabric is until you wear it and you won't always be aware that your backside is on view.