loose end drove me mad

Rating: 8/10 (12 October 2006)

I saw this last night and thought it was brilliant. Mark Wahlberg got my vote for his brilliantly offensive bad cop, to Martin Sheen's avuncular good cop. Leonardo Di Caprio can do no wrong and as the psychiatrist love interest said, his character's vulnerability is disarmingly attractive. The pace was great, the twists had me digging my nails in my partner's hand and even though I'm not a fan of screen violence, I was able to watch the whole thing.
But please can someone explain to me a glitch in the plot which left me really disatisfied with the ending. Why, when the girlfriend knows that Sullivan is a rat and has evidence to prove it, does she do nothing? She has nothing to lose at this point and exposing him would be the obvious course of action, particularly as she has fallen a bit for Leo. I just don't get it and it drove me mad all night. Please explain someone.