She is tall with short hair and a low voice and only wears trousers / shorts etc plus has always liked stuff that people (even in 2024) seem more likely to associate with boys - football, techie stuff etc. She regularly gets assumed to be NB or a boy and, if she says she's a girl, is often then assumed to be a trans girl.
On the whole, she's not bothered if people assume she's a boy and won't bother correcting them but quite a few times people have looked surprised / affronted to find her in a girls loo (or other girl's/woman's space) and she finds this embarrassing. She's asked me what to say to people when this happens. She doesn't want to be confrontational or make anyone fell awkward because she thinks it's not coming from an unkind place i.e. people are either genuinely confused or just trying to not offend. So, any advice on a quick way of knocking this on the head and not turning it into 'a thing'? Thank you.