<human>: I've been with my GF for 18 months, she lives with me part time. Up until this point we have had a solid/amazing relationship, with only a few minor arguments/disagreements, that have all been sorted out through healthy communication. The relationship has otherwise be great.

Last night she comes home and starts drinking, which we rarely do during the week, let alone a Monday night, however I agree to have a few to unwind, somehow she has a lot more than I, and she blacks out. Some she's never done before. I'm trying to take care of her, and I notice her phone keeps going off. It's a guy, who I don't know, but a name that I've seen her texting before. Someone she's described before as the 'brother of her close female friend, and a family friend for a long time.'

I've never snooped on her, or gone through her phone, because I'm a trusting person, However, while trying to get her into bed I oversee the messages, they're referencing going out to a "fancy dinner", "can't wait to do things to you", how bad she wants to "fuck him" so on and so on. She admits in her texts to him that she's drunk and wouldn't normally so that. My first thought is maybe that it's a one off drunk mistake, stupid text? My interest at this point is peaked, and I look back, and sure enough she's had similar discussions before, looks like they've even got dinner before, but it appears they haven't hooked up. Yet. 

I'm going out of town this weekend for work (military training) and she was planning to go out of town to see some of her girl friends. Turns out that seeing him this weekend was part of her agenda as well.

As much as it pains me (we had a solid relationship up until this point), I really should bail right? I'm so torn, but I feel like something like this should be a one strike your out situation right? Did I get lucky and prevent a mistake from happening, or did I get lucky and get a free out of a potential shitty situation?

Summarize the given text in a few sentences.
<bot>: Caught GF planning to cheat, already been out with the guy, was going to take it to the next level with him this weekend while I was going out of town for training.