ok,, and now the problem is that she lied to your face about the guy even though it wasn't against the rules in order to make herself appear trustworthy, and not upset you.
Little did she know that you crossed her boundary and betrayed her trust by sneaking into her phone. So you knew she was lying to your face.
Sound about right?
Ok, well she lied to appease you. How do you feel about that? I guess there was no boundary set to not to lie to appease each other. So Yes, you do need to tell her the truth if you had a problem with her lieing to you. Because this will fester in you and later on even if you can't put your finger on it you'll have doubts as to whether or not she is truthful with you.
But how does this conversation go? First apologize for not being truthful with her. You were wrong so man up and admit it.
baby I'm sorry, but I haven't been completely honest with you. I looked in your phone and saw a guy from work had been texting you. (She will be upset, specifically because you don't trust her, I don't know if she's mature enough to take into account that she in fact lied to your face.)
Let her rant about that without being upset then explain to her that you know it's YOUR fault for feeling insecure enough to check her phone, but it upset you that she didn't tell you the truth, and honesty is very important to you, and even if she feels like you might get a little upset it's most important to you that she tells you the truth.
Once things are aired out. Move on. No biggie. You'll need to look into why you checked her phone though, why do you think that is?
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"Keep your head up in the sky, you just a baby."
https://www.youtube.com/watch?v=HccL8jUIIWU - Adoration of the Magi Lupe Fiasco