Someone once said to me, "*****es will be *****es."
She was just being a girl. Girls "flirt" with other guys all the time, dude, relax and let it go.
Maybe, next time, if he's escalating it a little too far, step in and say something, but if it was just harmless flirting, chill out.
Furthermore, as to what you should do now, as long as you didn't get all pissy that she was talking to another guy and got all sour for the rest of the night, see what's she's up too.
I mean, really, are we so immature that we have to "punish" her (by giving her the silent treatment, cuz let's face it that's what these guys are suggesting by not talking to her) for talking to another guy? Get at her see what's shes up to, AND BE COOL!