Sounds like the kind of guy who should be ditched, but since you're going to visit him, perhaps wait until you're leaving.
Temperatures first. It can be quite warm here, especially in June and July, and rather humid (in the range of about 22C up to about 33 or even 35C). Right now, though, it's raining off and on and is supposed to do so for several days. It's also on the cool side, but still humid - maybe around 16C. So, bring an umbrella and be prepared for it to rain once in a while during your day.
My observations are based on what I see around me in my rather upscale neighbourhood, where there is a post-secondary business school, a high school and a branch of the university, as well as a lot of shops.
I think that the main thing you'd notice about the way French girls and women dress, in comparison with their American sisters, is that the French are more pulled together. They never go out in sports clothes (except to the gym), so no: a) sweatpants; b) Juicy Cuiture; c) tshirts or sweatshirts with sayings on them (nort even your sorority sweatshirt); d) big, white running shoes. Never. Not even to run across the street to buy a baguette. They also don't do a lot of pastel or bright colours (leave the baby pink at home). You see more blacks, olive greens, browns as base colours. They don't wear a lot of makeup, but they do frequently wear subtle makeup. No baseball caps, no polar fleece and try to stay away from sequins.
An easy day-to-day outfit would be a) a black or white fitted tshirt combined with b) cropped pants and c) sandals or sleek retro training shoes. Alternatively, substitute an A-line below the knees skirt for the cropped pants. If it's hot, you could wear a spaghetti strap tank top. Add a jean jacket or a cropped jacket and carry a real purse of some kind, not a a neon backpack. You're done for daywear. For evenings, bring a few dresses for going out. Don't wear jeans if you're going out somewhere nice for dinner.
Shops that exist in America that are popular here (and would help give you an idea of what French women buy) include: Esprit, Mexx and Zara. Also, watch the fashion show for this label,
http://www.comptoirdescotonniers.com (which is surprisingly popular with teenagers, young women and their mothers).
www.zadig-et-voltaire.com is a bit glitzier (maybe more LA?) will also give you a clue about what people might wear out at night. Agnes b. is an archetypal French designer; some of what's on offer at
www.agnesb.com is perhaps a little too thirtysomething for you, but it also gives you an idea of French style.
You don't need a lot of different clothes. French women tend to wear better quality clothes but wear them more often. Besides, you're travelling. So, pack some interchangeable separates and a couple of cute sundresses and wear them over and over again.
If you're here until the end of June, you'll catch the fabulous semi-annual sales, at which time you can buy a new Paris look.
