<human>: I'm a guy. We are both 25 years old, been dating for 18 months. My girlfriend's family is from Croatia, but we both live in the US.

The summer of 2004 after she graduated high school she went to Croatia for a month with her sisters. There she met a guy, and ended up cheating on her boyfriend who was back here in the US.

Summer 2006, same thing happened, now she is with a different guy, she goes to Croatia for a month, ends up cheating on this guy as well, with the same guy she met in 2004. 

Summer 2009 she was single but she spent time with the same guy from the previous 2 visits.

I found out all of this from her older sister (I play indoor soccer with her sister so we hang out once a week at least) and her ex from 2006 (who just started working with me). She doesn't know that I know that she cheated. When her sister told me this I just sort of ignored it until I met her ex. When her ex (the guy working with me) found out I was dating her, he mentioned it. I didn't believe it, but then looked through her photos on facebook from 06 and 09 and of course there are a ton of photos of them two together. 

I offered to go with her, she said I wouldn't like it over there. I asked why she'd go by herself, she said her parents/siblings couldn't afford going this year. I asked her to just got for a week or two, she refused, saying it's no worth spending plane ticket money and not going for 4-5 weeks. 

The worst part of it all is that the guy she spends time with while she is there lives next door to her grandparents (where she stays while there). There is just no way that she will not run into him, spend time with him and possibly cheat. Should I just call it off now and just avoid the possibility of getting cheated on?

Can you condense the text into a few key points?
<bot>: Girlfriend cheated on previous exes with the same guy, now she is possibly going to spend 4 weeks with him.