Well personally if they told me they had been with someone else whilst in another country, then came back and said it didn't really happen, I would still finish with them as A) They either really did do it and are now lying or B) They didn't do it and they are playing mind games and I ain't interested in being with someone who would try to hurt me like that.
Ask yourself why would she say she had done it and then come back and lie? There's obviously more to it that only you know about (maybe mind games is a common thing with the relationship? or you were going through a rocky patch and she was trying to make you stand up and notice?)
You need to think seriously about why she would say these things, maybe have a good talk with her and decide if you want to continue the relationship or not.
Good luck :o)