I don't have much experience in this realm, but I'd have to say, you need to tell her honestly about how you found out.
You can't live with the fact that you are expecting her to be honest and you can flat out lie and tell her that someone else tipped you off about her cheating.
It's gonna be a rough one for sure, but if she's really into you and you are for her, you guys should come out of this fine.
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I'm so in love with a girl... she is my everything
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