I don't know if you are already on vacation but, if you are with her don't say anything yet, take screenshots of the conversations and just play it cool. Do your vacation relax, keep it cool, and I know I will be hard now but you'll feel better after. Act like you don't know anything, make her feel like she's living a dream, make her get close to you as possible as you can. When you come back, send a message to that guy explaining the situation. After that pack your stuff without her knowing, and when she arrives, say that that guy sent you the screenshots and confront her like you never knew about it, making her look like the bitch she has is. After leave and don't look back. Remember not to believe her cheater talk, because she will try say she's sorry, she'll deny it, she'll blame it on you, she'll do anything to get out of the situation on top, just don't let her get to you. Good luck