In a scenario like this, you'll naturally be tempted to give your former lover turned friend-fucker a phone call. The conversation would be a pathetic mix of expletives, death threats, unbecoming wining, and angry accusations about "forgetting the beauty of what we had." Resist this urge.
Aside from being an easy way to let off steam about the situation (there's something strangely therapeutic about yelling, "DID WE MEAN NOTHING?"), doing so will make you look like a hard-on harboring pansy who is either still in love with her, or unable to put his wounded pride in check. It's safe to say neither are particularly flattering.
Rather than giving her a chance to feel smug, don't give her a reaction. Period. Hell, it might be the only reason she’s doing it in the first place. Women have been capable of more manipulative schemes. Believe that.