Should I break up with my girl friend?

Me and my girl friend been going out for 4 years now. I recently found out that she has been talking to one of my close friends who is 33. Me and my girl friend are 20. Everything thing was going perfect until I went overseas and when I come back and just randomly one day go into her cellphone account and find out that she has been talking to this guy for couple hours a night. And they also been calling each other back and forth during the day as well. When I asked her about this at first she started crying and said that she couldnt believe that I thought there was something going on between them two. She said she just talked to him because he is a good friend and a good listener which he is. Then I asked the guy the same thing because 3 months ago when our relationship was going down the drain the same guy had try to fix things between me and my girlfriend. When I asked the guy why he did this he sweared on his 3 daughters that he was just an ear for my girlfriend and he considers me as his younger brother because his biological brother passed away 2 years ago. So had been talking back and forth for last 3 months. My girl friend still talked to me normally and everythign seemed the same until I found this out. I was being jerk with her couple months before this. And she was also going through stress at school, home and work. So she felt that there was no one there for her so she went and talk to him. What do you think I should do? should I try working things out because it seems like she still cares!!