My friend has been rejected by his crush (who is my best friend)?

Try to act like everythings fine, just remain happy. Don't talk to him about it unless he comes to you for help. Don't feel guilty, it was much better him finding out for himself then you telling him, and theres not always going to be someone to tell him when someone is going to brake up with him. You did the right thing, now he knows how rejection can feel and can gradually get more confident. Anyway it would have been harder for him to hear it first, and then again from her.