Help on meeting my ex's girlfriend?

Soon i'll be going to a gig because my friends band is playing but my ex's band is also playing so his girlfriend is definitely going to be there. I have to go to support my friend because it's his bands first time playing, but it's a small thing so i'll definitely see her. We've never met before and me and my ex broke up about 4 months ago and he's been with her for about 3 months. When they got together she sent me some nasty messages about them being together now so I wondered if anyone could give me any advise for when i see her? I originally wanted to confront her about the messages but my friends have told me not to 'stoop to her level'. Any advise is greatly received!