I think honesty is your best bet here. A true friendship will last through anything. A true friend you can piss off and have a disagreement with. You sound like true friends to me.
I have been w/my hubby for 2 1/2 yrs now. We were friends for 3 1/2 years before that. We met in high school and clicked, just like you two. There was always something different about him. We fought, we hung out. I was with other people and so was he. I was into drugs at the time and really wasnt ready to give myself to anyone. He knew this and never tried to push himself on me, but I always new that he felt more for me. I just couldnt give anything at the time. So we remained friends and wrote each other while he was in jail and while I was doing my thing. But through all this that connection we had stayed strong. It was a friendship stronger than anything I ever had. One day after we graduated and became responsible adults I kissed him. I was ready for him. It was like a light came on. And I realized everything I wanted in man was always in him. We have been together ever since and are having a baby soon.
I guess when I look back on it he always meant more to me than I let on, I didnt even admit to myself. Now he told me that if I had waited any longer he may not have been around. he loved me for 3 years from afar and he had given up on the hope of ever being with me. I always knew that he like me more than a friend, but I had no idea that he loved me. I think if he would have told me his true feelings it may have been different.
So tell her when you think the time is right. I knew when the time was right. Maybe you will too. But remember that love can be lost and you risk losing it if you dont grab onto it. She may feel the same and she may not. But you should tell her how you feel about her. Dont pressure her and dont try to make a move. Just be honest and let her figure out the rest. Like I said before a real friendship will make it through. It may be akward and reallu weird at first but if its meant to be it will be