Or has she been the girl not saying a single word, being visible for about five seconds, sitting next to Robert Gant when he met Jeremiah and Vinn in the restaurant?
Crediting her for that is ridiculous. I guess there have been more scenes which have been cut. Are there deleted scenes with her on the DVD?
You're right, Sandrine is the girl sitting at the table with the main characters in the flashback scene.
There's been a lot of discussion on this board about why she's credited at the start of the film, and we've generally reached the same conclusion you have: originally she must have had more scenes that ended up on the cutting room floor. Presumably her name remained in the opening credits due to an oversight, or because she's got a great agent.
Regarding the DVD, I can only comment on the Region 2 disc, which I own. It contains no deleted scenes, only the trailer and a ten minute HBO feature about the film. Sandrine does not appear in the latter.