Opposite sex friendship or just a backup plan?

I've been dating an amazing girl for a year and 3 months. She asked me to fix her iPhone recently and I found some texts from a guy that were really personal. He said he wanted to marry her. She sent him flirts which happened to be the same ones she sent to me. I confronted her, and she said he was a great friend of 6 years. They did date a long time ago, but she did not sleep with him, and she is not interested at all. I also found out that anytime she said she was going out with "a friend" it was him. A week ago she told me she wanted to goto a neighboring state to just get away. I asked her who she was going with. She said by her self. I told her I wanted to go with her. About an hour later she requested that we meet somewhere to talk about something. She admitted that she lied and she was going with him. I have some really deep friendships with the opposite sex, so I want to give her the benefit of the doubt, but am I just being stupid? What should I do? If it was some run of the mill girl, I would dump her, but she is really amazing.