It's a tricky line OP - I used to work with my sister and was friends with her assistant.
DSis had to speak to her a couple of times about appropriate office dress and makeup, as the assistant was a fan of plunging crop tops, and was plastered in makeup but several days old eyeliner all over the place and hair unbrushed (not a fashion look, just v messy).
The assistant took it as bullying and considered reporting her to HR.
Knowing both of them as I did, I can guarantee my sister did NOT bully her, but would not have backed down when disagreed with, and may have chosen her words badly.
The assistant did actually look too tarty/unkempt for her role, but couldn't see it, and was offended by the suggestion.
I am very glad I kept out of the whole discussion!
If you are going to approach her, I'd do so with a freshly written company dress policy in your hand, as you distribute it to all staff, without directly criticising her clothes.
Bear in mind tho, a new work wardrobe doesn't come cheap, you may need to give her a while to get it sorted!
How about:
"Smart business attire to be worn, jackets and ties preferred for men, minimal makeup and jewellery, no denim, sportswear, or casual footwear (trainers/flipflops)" etc.