I have been friends with this girl for quite sometime now. But she has a boy friend. Know her boy friend is one of my other friends. He betrayed me cause he knew that I was proper close to that girl. Also he kept complaining every time he saw me with her. Then one day he went to ask her out. That broke my heart. I still have feelings for her but how can I tell her that I love her.

Of course you can, that is if because you love her. There is no reason to stay secretive about this, and if your friend gets mad, well he has no right to get mad at you since he knew you love her too.
Be careful though, because I don't know what type of person is your friend, so I can't assume what will his reaction be like.

How old are all of you? I have been on both sides of this while dating a girl my friend liked, and it was awkward for him, seeing us together, and also have had to see a friend of mine date a girl whom I had a rather strong crush on.

In your case, back away from your friend and the girl and seek other friends and activities for a while and let them have their fun and you find your own to have. No you don't go telling her you have strong feelings for her because she has someone else for now.

Play this real cool with no hard feelings and don't beat yourself up, or hold a grudge against your buddy. It could easily be him on the outside looking in, and how would you want him to treat you?

Put your best foot forward at all times and look around at other things to enjoy. Hey, she likes him, so let her enjoy the time. That's what her REAL friend would do, right?