Hmmm...
I wouldn't say anything, because that could hurt your friendship with her, as well as her relationship with this other guy. You have to be careful about what you say, because you don't want to become the guy who just puts down whoever she is with.
I was in a similar situation with my last girlfriend. I met her while she was dating another guy. She had been dating this guy for 4 months, so they were no where close to getting married, which makes my situation much different. Anyway, I didn't give any negative imput about him to her, and just listened. Before I knew it, she broke up with him, and began dating me.
What I am trying to say, is that saying bad things about her fiancé isn't going to get you anywhere.
However, as other members said, maybe just telling her how you feel could work.