Should I forgive her for cheating?

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So my gf went on a trip to her brothers place in a different country for 2 weeks. The problem was, that her brother was living with another guy. Because of that, there wasn't any available spot to sleep except on the same bed with that guy.
I was realy frustrated about all this, but at the first day she convinced me, that she loves me and nothing will happen, ever. That she has no appeal to him ect.. My intuition was telling me that something will go wrong, and of course.. It did. At the very last night, before coming back to her home country, she got realy drunk with the dude + smoked some weed. Well she was wasted hands down, and at the end of the night, she had sex with him.. She explained to me, that it ment nothing to her, that she couldn't control her thoughts because she was really wasted, and one of the reasons she did it was, that this dude was pressuring her to do it for the whole 2 weeks and that I wasn't there for her( I wasn't near her to remind her about us). Oh, and I found about this myself from hacking her FB account, where she was chatting, sending letters to the dude about how they should meet sometime, when he comes to her home country ect.. She said, that she was just using him, that she didn't mean anything she said to him.

She cried, begged me for giving her another chance on the phone for 6 hours during the night. She said that it won't happen ever again, and that she knows my trust is lot for her, but she hopes she will regain it during the time
She said, that she loves me, and I mean the world to her, and that she is scared to live without me.. After all that I sort of said - "ok, we will see", which ment I gave her a small chance, but I'm not sure, if that was the right thing or not..
Oh, and she said that she will stop all contact with that dude, and everyone else that I see suspicious of threatening out relationship.