sama, if I were you and If were not able to find peace. I would yell at Hubby, and would confront the girl. I know its not logical but I would get you anger out. I mean whats your is yours.
If he was treating his ex with respect and was not being chummy with her then this is not the way to deal with this issue. Even if he was doing something naughty you do not create public scene at a party. There are more civilized ways of dealing with the issue in case that is the issue.