You've already crossed the threshold and found the evidence. Was it a slightly crummy thing to do? Yes. However, let her know that you have the info, and it's game over for her.

A little piece of advice: don't let this experience sour you or put a bitter taste in your mouth about women and relationships. Not everyone is going to cheat on you. You've learned a valuable lesson here. You learned to trust your instincts. Take that lesson and the other positive things you've learned from this relationship and move on. Best of luck.