What a fun problem
As far as dress code, that depends a lot on what the office environment is like. I would think a polo shirt would be for casual fridays only - but that is a good question for her to ask them!
My first inclination would be to get these things:
pencil skirts: one black, one color/pattern (but not loud)
pair of pants, neutral colors (black, grey, tan, etc)
button down blouse/dress shirts: one white, one color
sheath dress, grey or tan
different color cardigans: one neutral, one color
work appropriate shoes if she doesn't already have them (Closed toe)
belts to match if needed
These can go with anything she already has in her wardrobe and should give two weeks worth of outfits w/ accessories (scarf, jewelry, etc). A blazer doesn't hurt either but might not be necessary.
So, possible week's worth of outfits:
Monday:
pencil skirt, long sleeve shirt/top, cute headband or other accessory she already has
Tuesday:
grey or tan pants, button down shirt tucked, bright color shoes (closed toe) and belt
Wednesday:
sheath dress with cardigan and scarf, belt worn at waist over dress and under cardigan
Thursday:
pencil skirt with button down shirt tucked, statement necklace, could add a belt here too
Friday:
pants, top/cardigan, cute headband
You'd only have to buy one pair of pants, one skirt, one dress, one cardigan, and one button down to do that. Just add fun, bright accessories and she can still look "her age" and be office-okay.
Target has some GREAT stuff in this category that is dressy enough for office but isn't made from high maintenance fabric - a teenager is not going to get her clothes dry cleaned. Good prices, too, obviously. I think you could easily manage the list above under $200 at Target and the colors would all coordinate.
Banana Republic also has some good things in this category and they often have good sales. Old Navy will have some things too, but might be hit or miss with the more office-y stuff until fall.
So sweet of you to take her! You'll have a blast.