You are over thinking this. She doesn't need to look individual or express her personality through her clothing.she needs to fit in. And she's not a child, she's a young women. Many of our parents generation were out earring a living at 15
A darkish plainish dress/smart trousers/knee length skirt will be fine.it doesn't t need to be black, dark grey,navy, aubergine etc is ok . Team with a smart ish top or blouse. Not a t shirt or jersey top ( like the next one you linked to )
Subtle make up, discrete jewellry, no short skirt or see through blouses. No underwear that shows through clothing. No bare legs . If its hot on the tube she can take them off before her journey home
No one ever didn't get a job in the city because they looked too consevative or boring.
She needs to stand out by her behaviour, not her clothes. She shoudl be very polite, listen well, ask intelligent questions, be willing to help, show enthusiasm and NOT LOOK BORED even if she is.not complain. Be early every day. Not continually ask when coffee beak /lunch is. Not use her phone or even consult it in the office.not discuss any aspect of her private life at all except her education and her career plans. No talk of boyfriends,friends, shopping, parties, getting drunk , hair , make up.
These things will mark her out from 90% of most other work placement students .