Dear LTV,
Believe it or not, many students find themselves in situations similar to yours sometime in their pre-teen or teenage years. That said, each situation has its own little details and nuances, rendering the advice I give different each time. In your particular case…I would talk to your girlfriend about it; be honest with her about the entire situation! You referred to her as one of your “best friends”; if you are that close, I don’t think a guy would be enough to permanently jeopardize your friendship. She may still be hurt and resentful—jealous, most likely—for a week (give or take, depending on how much she likes this guy), but if your friendship is worth anything, she’ll come to her senses and those hard feelings will pass.
If you really can’t bring yourself to tell her, your response and reaction depend on how much you like the boy. You could tell him you can’t and won’t go out with him in as kind a way as possible, and then gauge from his reaction how serious his feelings for you are. If he’s a regular pleading Romeo and you really have liked him all along, don’t leave the guy hanging—remember that it takes serious guts to tell a girl to her face that you like her. If he shrugs it off genuinely and you aren't heartbroken…well, try to let it go, and maybe even drop some hints to help set him up with your friend.
If you do decide to go out with him, do NOT hide your relationship from your friend—she’s bound to find out sooner or later, and her sentiments of betrayal will be doubled by your secrecy, whether it has good intentions or not. Be as civil as possible to both parties, and write to me anytime with further concerns and dilemmas. Best of luck!
Agony Aunt