GF Texting Ex about Sex?

I'm going to start off by saying that I know that the way I went about finding the information was dead wrong. But I know it now, so here it goes...

I looked through my girlfriend's phone tonight and found a text conversation she had with an unsaved number some days ago. I believe it's her ex because the conversation included how proud she was of him making new strides, and because the person had a nickname for her that sounds really familiar from her past stories. She texted him saying that she was stalking his Instagram page and said that he was "so fine." He responded by saying her new hairstyle had him thinking nasty thoughts. She said that she was imagining what it would be like for him to bend her over...and the conversation goes on, with both of them expressing a light-hearted, joking disappointment that they couldn't fulfill their fantasies. I come up in the conversation, as she says that I'm a "good guy, but it doesn't stop me from fantasizing."

I don't know what to do. If I tell her, it'll be clear that I've been through her business. And this ex lives four hours away, so I can't imagine them getting together behind my back. But how can I not address this? We've had conversations about how annoyed she was that she kept getting contacted by exes, but she's CLEARLY doing nothing to keep this one at a distance. And for her to give him the satisfaction of knowing that he still has sexual influence is killing me inside. What do I do?