If it bothers you, Dump Her.
Before going on to explain why such a trivial complication should be resolved in such an extreme and abrupt manner, we need to explore the idea of “cutting losses” and why it’s such an extensively circulated solution among the game community. Almost 90% of the time you have a problem with a girl the immediate solution offered by any seasoned Don Juan will be “break up with her.” It’s not because we’re all unemotional cyborgs, the answer has more to do with efficiency. If 100/10,000 girls are “good matches” for you, then the real danger is NOT in missing out in your “soul mate” — it’s wasting your life away with a girl you are NOT a good fit for.
There is no such thing as a soul mate — only a good mate. Someone you can work things out with, someone who has enough natural chemistry with you to compensate for the problems that plague every relationship. If you dump a girl “too fast,” one who was a good fit for you, the consequences are relatively insignificant. You will find someone else who can make you just as happy or possibly happier; assuming you have enough ambition to pursue another relationship. If you allow yourself to get duped by the soul mate propaganda war, you will not find someone else because you aren’t allowing yourself to.
On the other hand, if you mistakenly AVOID dumping a girl you AREN’T a good fit for, the consequences are far more severe. The monotony of a loveless life co-inhabited by a succubus will compulsively strip your existence of all color. But even worse than actually living through such an ordeal is the nagging voice in the back of your head constantly telling you that you might have been able to live a better life if you had the courage to overcome your fear of loss. The unhappiness from a break up with a “good fit” will be temporary; it will last until you find someone else. The unhappiness from staying with a “bad fit” is eternal. It is a mistake that cannot be fixed.
This precept might seem vividly obvious when I put it down on paper for you, but rarely do guys abide by it throughout their lives. Sometimes the whole “soul mate” myth envisaged by the constructs of a feminine lake is simply too lucrative to keep people from prematurely cashing out. The illusion of security offered by a relatively “stable” relationship — no matter how unstable it actually is — can be a narcotically delusional opiate.
The scary thing about life isn’t not being able to live up to your FULL potential; it’s not being able to live up to any potential.
Now on to the question at hand. There are two possible ways that your girlfriend’s best friend ended up being a guy.
(a) She was best friends with him before you guys were going out. He might have been a past flame or a nice guy trying to find some backchannel to her figa. Either ways, as a corollary to the above, the only reason you would want to put yourself through this is if you were desperate and wouldn’t want to pursue another good fit without this caveat.
(b) She became best friends with a guy after you guys got together. In such a scenario, clearly you fucked up somewhere or she’s tooling the guy for shit testing ammo / ego inflation. If this problem bothers you enough that you need to rethink your relationship, you’re better off starting fresh.
This is NOT to say it’s intrinsically WRONG for a girl to have a guy as her best friend. If you have no problem with such a scenario, then don’t worry about it. Personally I wouldn’t care if my girlfriend’s “best friend” was a guy. But if such a stipulation DOES bother you, you now know what to do. Don’t try and figure out some cheesy method to break up her friendship, just move on.
For academic purposes only, I’ll also add that jealousy is always the best solution to jealousy. If she’s getting really close to another guy, you should be getting really close to another girl.
I say the above for “academic purposes” because it’s how you would “hypothetically” solve the above situation without a break up – although it’s not the most optimal solution. For long term relationships with good prospects or marriages, I would take this route instead. Avoid appeasing, compromising, or “talking to her about how you feel.” It won’t help, I promise you. The whole “tell her how you feel” idea is a justification AFCs use to vent about their insecurities. You should communicate with your woman when you need to, but don’t whine about your anxieties.