That's why it's so tough to trust her now, yeah? Because she said she did it once and lied. So now, you're in the hellish position of trying to trust her again, or checking her phone to make sure she's telling the truth, which is possessive and unhealthy itself.

God people who do this suck. I don't get the people who don't just do what they say they're going to do. It's so hard to trust someone again after this. Good luck in whatever choice you make. In my opinion it's about equally valid to stay with her or dump her at this point. Just remember that if trust doesn't come back eventually, or if she leads him on again, it's better to break up than resort to zero-trust stuff like phone checks or suspicion.