Ok so I have two theories, the first is that she cheated for real, and it was with more than one person. The reason for this would be that on the fishing trips it is implied that it is mostly male with one female they fawn over which she might have interpreted as him cheating on her with his sex group thing which in turn prompts her to do the same to him as a way to get back at him.
The second one is that she faked the cheating maybe as a way to get him to break up with her and be done with relationship, but it didn't go the way she intended which is shown by how he clings on to her later on and has sex with her one sided-ly, and so she finally knows that the only way to end this is to leave for good.