Break it off with the other girl before you try to do anything with this girl. The only trick is, if she isn't interested in you, then you will be out the other girl as well. Its a risk, but life is full of risks.
I had a friend who I was interested in. She and I were good friends, but she was seeing someone and so was I. A few years later, I had another opportunity, and didn't realize it until it was too late. We are still friends, and we both are seeing other people, but we both have talked about what could have been. I have no regrets and neither does she, but we did wonder why we didn't try before.
I would let her Know how you feel. If you don't want to break up with her gf yet, then you should hint around to the friend that you like her and see what happens.