I was in a similar situation not too long ago. My b/f of 3yrs was not there for me as much I needed towards the end, and I met this guy through a coworker. He was there everytime my b/f was busy or wouln't answer the phone.
I also told him that we were just friends, that he was just someone I talked to, but before I knew it I started to like this guy a little more than I should of.
I started arguing with my b/f all the time and he became very jealous. The more we argued the closer I got to this guy.
I didn't realized at the time that my b/f was right, that I was the one causing the problem.
I'm not saying that your girl thinks the same way, but we women are much more emotional than you guys. We look for emotional confort and attention.
Talk to her and ask her what she needs from you. What she would like to see differently. If she's happy with you and you give her everything she needs, and you make her feel special and loved, like she's most important and most beautyful girl in the world, then she wont need to spend so much time if any with her guy friend.