You don't.
Then i guess shes not your best friend because a best friend should not be going with someone that you like especially if you told her!!!!!!!!!!!
if there is a chance of your friend being with their crush back off if there is not or only a small chance or their crush likes you then tell your friend i told my best friend she was fine with it i mean its only a crush its not like they are actually dating
If she knows you like him then she's not your friend and you should find a different crush and a new FRIEND! But if she doesn't know tell her and if she doesn't understand or doesn't care then find a new friend and once again a new crush
Well it look like you and your crush are over - sorry.
don't act like you like him !!
If your best friend doesn't know you like your crush than tell her how you feel about it. what you can also do is tell your friend that you liked him first or tell him to decide between you two. or you can also share him. or you or your friend can move on.
Wait
You must make a decision on who you like more, and find a nice way to break it to whoever you don't like.
well if your crush would do that he/she is not who you thought they were so you shouldn't like him/her anymore
Tell your friend that its not nice to date the person who you like and have know forever, and you don't want to be friends if she/he is going to be like that!
Tell them might be a good strat but you might want to give them signs.
First I would figure out why your best friend did that. Then I would tell your crush what happened and that you really like him.