I don’t know how old you are, but I’m guessing you’re at school. And I’m guessing you and your friend are generally together. Which means every time you look at him and he’s looking in your direction, he’s probably looking in her direction too. So if she was to turn and look at him, she could think the same thing.
I think you should ask yourself if you’re willing to risk your friendship at this stage in your life. School romances rarely last long. If you’re sure he has no interest in her, there’s no danger of them getting together and leaving you behind.
I hope you have other friends who will stand by you if you really like the guy and you’re willing to risk upsetting your friend.