<human>: Can you summarize the given text in a few sentences?

Back in November my girlfriend had a bad night involving alcohol and an argument with family that ended in her going to a friends house, making out with him, and passing out in his bed. She apologized and confessed the next day and our relationship continued normally. 

Now she is abroad for a couple months and her best friend came up to me and revealed that she was there that night and that wasn't the whole story. There was more that happened in the bed and possibly more events in the relationship i am being lied to about. 

I got the phone number of the guy she cheated on me with intending to text him. I facetimed with my girlfriend to ask what really happened and she said that what she had said before was 100% true. I wouldn't be mad that it happened especially considering the circumstances, I'm mad that she spun this web of lies after in order to make it seem like it didn't happen. How do I find the truth? Does the truth even matter? I can't break up with her if i don't have proof, and he will likely deny that it happened because she will have told him to not say anything.
<bot>: Girlfriend half-confessed to infidelity and I don't know how to treat her or how to catch her in her lie