If she wasn't doing her placement at your place of work presumably she'd be going to your husband's or elsewhere, and your feelings about her personal hygiene would be the same, wouldn't they?
She's not alone in her grungy habits; I've heard of plenty of teens of both sexes who go through a phase of neglecting the basics.
I guess you'll be ensuring that she knows the basics of what sort of behaviour is expected when going to work - being on time, doing what is asked, not texting friends whilst at work, being pleasant but not over-familiar with co-workers and so on? Why don't you reiterate all of these and add in the clean clothes, clean body, clean hair, clean teeth aspects?
In a way, it's a pity that it's just a one day placement - a week or more of higher standards might encourage her to take a bit more care with her personal hygiene.
i have 14 and 13 year old boys and I can assure you it isn't high on their list of priorities either, but I still tell them to shower and wash their hair and brush their teeth every day and they do it. They wouldn't care or remember without me telling them. If the dirty clothes aren't put in the basket then I just take them out their rooms and they can't then put them back on.
I appreciate that they need an element of independence and privacy, but at the end of the day you are still their parent and they are still your child. There is no way on earth that i'd allow them to embarrass me or themselves by being dirty.
Glad that it is sinking in, it's a long hard slog. Mine have been getting spots so we have had a chat about the need to keep their faces in particular cleaner. They are liking a bit of attention from me helping them apply cream etc. Maybe something like that would work? Would she like a bit of tinted moisturiser and you could show her how to apply it but it has to go on a clean face. Maybe a bit of fuss over her hair too? Apologies if you've already tried that.