My husband has Grand Theft Auto: Chinatown Wars for his DS and LOVES it!!! He can't put it down. If she's looking for involved, detailed, and adult-themed, that fills the bill. I haven't played it myself but he thinks it's one of the best ones ever made for DS.
As racing games go, you can't go wrong with Mario Kart DS. Always a solid choice.
Personally, as an adult female, my favorite games for the DS are Tetris, Space Invaders, Rhythm Heaven, and Big Bang Mini - obviously I'm more into simple quick puzzles so that might not be her preference. I also use a LOT of the educational/application type "games" like My Spanish Coach (and French, Chinese, Japanese) and I LOVE the new My Healthy Cooking Coach, which is basically a DS recipe book. It's similar to Personal Trainer: Cooking, but I like it a little better just because the recipes are more geared to my taste.
Too bad she didn't get the DSi! Right now I mainly play games I downloaded for it, just love Dr. Mario Express and Sudoku Master!