Can you clarify a few things because I’m a little confused.
First of all, are you a female? Is your best friend a female too?
Is it you who has a crush on the guy in question, or are you falling for a guy that your best friend already liked? Do both of you (girls I’m assuming) both like the same guy? Does the guy like or have a crush on your friend, already?
If your friend told you that she liked this guy (let’s call him Max) first then you should probably just be gracious and not interfere with her attempt to date him. If you jumped in too, it would be really mean to your friend.
If, on the other hand, you told your best friend that you liked this guy first, but Max, for whatever reason is interested in her, then you need to forget about this guy, and hope that your friend doesn’t go for him. But if she does, know that your relationship with both of them will change, but if you’re gracious to them both, you will be a very wonderful and strong person, although it might be rather painful to you for awhile.
Whatever you do, try not to beat yourself up if you have feelings for someone (Max) who is not available to you, but don’t make your best friend feel crappy by suggesting that your friend tried to steal him from you (even if she did) because it will make you look bad. And don’t try to steal Max away from your friend if they end up together, because that will also make you look bad and make both of you feel horrible.
These kinds of situations are really horrible, and they tend to happen more, when you’re younger and still in school, because you and your friends will tend to like the same exact things and not be able to even realize other possibilities. When you get a little older, even though you may have he same friends (and make new ones too) your interests will become more diverse and you will be able to see possibilites that you may not have even considered before (like other nice guys). Good luck to you : )