What is your friend's budget? That will have a major influence on what fabric you can work with.
If she's short on cash, she should either (a) wait and save up more money, or (b) be prepared to be satisfied with using basic cotton, which can look flimsy, thin, and limp quite quickly. Avoid cheap costumey polyester satin.
If money is less of an object, there are several ways to go: a low-luster/non-gloss high-quality polyester satin ("casa satin" or duchesse satin, often found with bridal wear) would give a polished and leisure-class look. For a more sturdy garment that a character could use for everyday wear, wool suiting or poplin would be perfect. If your friend wants to splurge, you can price out some of the stronger silk fabrics.
What you go with hinges, really, upon two factors: (1) How much is your friend willing to spend, and (2) What is suitable for the character? I know nothing about the character, so it is difficult to tell whether she's a refined lady (which would use an expensive, high-quality material like silk satin), a lower class woman who is splurging after becoming suddenly part of the nouveau riche class (who would likely use cheaper but flashier fabrics), or the nicest dress in the wardrobe of a sensible, responsible, middle class woman (wool and poplin). Just some food for thought.
Pictures and tutorials from my costuming portfolio at Syagria's Cosplay Site!
Star Trek Original Series Uniform!
Original Creation for AB13
Most Recent Con:
PAX East, Anime Boston 2013