I'm going to a leaving party next month as one of my old high school friends is jetting off to South Africa for 6 months. It's a good ol' high school reunion, which should be fun except for the fact that one of my ex's and her current boyfriend will be there.
Just avoid her and talk to the people you're friends with. There was a time when relations between me and another girl had become pretty bad (after royal fuckup in attempting to make another pass at her after being rejected a month before), but whenever we were at events together, I just avoided her and talked with my friends and other people. It worked out great, and I'm pretty sure both of us had a good time, despite not being comfortable around each other.