Depends on how hot your girl is. If she's smokin', you can bet your a$$ that if the opportunity ever came up, that guy would jump on it faster than lightning. The girl, however, is less likely to do that, but as I said, their history will tell you most of it.
I've been on both sides before. I've been the BF with the girl who has a close guy "friend" and there was supposedly nothing going on. But at the same time, my girl was not some super-hot model to the point where any guy would jump on her. This was my very first GF, so I was pretty much getting started with the whole relationship thing, so I didn't really care anyways. I sort of knew we wouldn't last super long, it was just for experience. Anyways, back then I was not too comfortable with this guy around, and it led to some disputes. No longer is that sort of thing happening, but you get smarter with age.
On the flip side, I've also been the guy who the girl is friends with, and the jealous BF gets all ticked about. This is more fun
heh heh. Well, anyways, I'd say in general, there is not a lot to worry about, so long as the guy respects the boundaries. If he's playing your girl and actually pulling moves on her, then maybe start worrying a bit. The reason for this is that it may come down to the point where the guy actually makes the girl see what she's missing out on and she'll start re-assessing your relationship with her and start comparing the two. That will make your life more difficult because you'll be thrown into the inevitable position of having to either step up to the challenge and give her a reason to stay with you, or else risk losing her. I know you don't want to hear this, but it's the truth.
I know a girl now who I am STRICTLY friends with. There was never any attraction between us, and if anything, it was her attracted to me. She has a BF and they've been together for like 5 months. She tells me how she met this other guy who's everything she wants, and she's just so confused as to whether she should stay with the BF.
The worst part for the BF is that she's actually to the point where she's finding reasons to justify NOT staying with him. That's rough. I'm sort of acting as the emotional tampon, which is easy because I'm not interested in this girl at all. It's quite an eye-opening experience, and goes to show you that don't believe this friendzone stuff is going to be some absolute BUFFER to her getting with the guy. Anyone who thinks that anything is absolute in this case had better take their blinders off. Relationships are ever-changing creatures. You have to always stay on your toes, that's why you need to make sure you can spend the time before you get into one.