Honestly if she's your best friend then she should know who you like so this stuff doesn't happen... And when it does it starts all types of friction between friends. So there's that part the way to talk to her and tell her is the same way you should have told her when you discovered that you have feelings him BUT if he said yes to her and they are happy then you can tell her but also do be the pain that breaks them apart. It's one of those you had a chance and missed it type of things.
I like a boy in my class, but today my best friend asked him out and I don't know how to tell her I like him without hurting her feelings. What should I do?
Tell the bff before you see them making out in the hall that it breaks your heart!! If you really like him you'll tell your bff, and if she's really your bff she'll break up with him... But soon after she breaks up with him you need to swoop in and take him or your bff'll get mad!