You have a few options. If you want to remain friends, you stay away from him. If you want him then your going to loose your friend. The only other option is to tell her you are attracted to him and ask her what she would think if you two went out. When I was younger, I faced a dilemma similar to this. My friend liked a guy. They weren't together but she saw him first. I made out with him at a party and lost my best friend of 13 years. I'm 39 and she still wants nothing to do with me. It wasn't worth it. I miss her everyday.