If you truly care for your best friend and love her like a sister, then don't do it. Imagine it the other way around, if you were the one with the boyfriend that your best friend is in love with....in my book, not cool.
I was in a situation, well a similar one, where my "best friend" started dating my ex (but the story was complicated, since I wasn't over him and my "best friend" knew that), but that was not the worst part, she was actually hiding it from me, because she was just "so in love".<br />
First of all, what does the guy think of you? Does he feel like you do? Or he doesn't even know. If he doesn't know you have feelings for him, then back the hell off. Don't tell your best friend about those feelings and don't do anything with the guy. BEST FRIENDS DON'T DO THAT!<br />
But if the guy knows and acts on those feelings while still being with your best friend then remember: if he'll do it with you, he'll do it to you!<br />
Either way - you would lose your best friend, who then in the end even wasn't your best friend.
If you can get him away from her, go ahead and do it. Better for her to know now that neither of you are worth her time. Now, before she makes a baby or two with him and then discovers he is worthless. <br />
If you can take him, he isn't worth having and neither are you. So, I say *Go for it* and get whatever you deserve. Either way, if you get him or not, you will lose her; because if he isn't interested, he will tell her and she will drop you like poison ivy.<br />
Oh, and by the way, if you can take him from her, just remember that someone else will be able to take him from you just as easily.
Talk to her about it and maybe have a poly relationship
If you have any self respect...You won't do a damn thing about it. Too many men out there for someone to backstab a freind.
talk to your best friend about it. try not to sleep with the guy.
dont be a drama queen