Lexi D. wrote an answer about Rossignol Sassy 7 Ski - Women's on October 2, 2014
Hello Free Heeler!
170cm sounds good for your cousin at her height and ability level. The rockered tip and tail remove a bit of effective edge from this ski, making it ski a little shorter than the 170cm measured length. Rossi will take into account a female's weight and lower center of balance when recommending a mounting point, so I'd stick with the factory recommended mount line.
I'd also consider something from the Blizzard line, they make amazing, stable skis that are great for both East and West coast skiing. I'd check out the Black Pearl which I've heard tons of intermediate skiers RAVE about as they find this ski helps them easily progress. I've thrown in another really great option from 4FRNT. I wrote a super detailed review on this one for Blister Gear review, that link is included as well.