Ok I've been through the exact situation you are speaking of.....with one exception. I was the guy that you are friends with. (not literally obviously, but in that position) I was best friends with a girl named Jayme for 3-4 years. Close as hell. She knew how much I loved her b/c I would let her know as much as I could. She had a boyfriend for like 2 years and when they were having problems I was there for her. Eventually they broke up and we started messing around and things got real serious real quick. I can honestly say out of all the times I've had sex in my life, with her, it was the first time I really MADE LOVE. Anyway to get off that, she ended up going back to her x and she really hurt me and it's been over 2-3 years now and we never really spoke again. And like I said this was my best friend of over 3-4 years. It was all thrown away for 2-3 weeks worth of what I THOUGHT was love. She was just confused and I got hurt bad b/c of it. My advice to you is just be sure you are over with your man before you do anything with him. Sometimes as hard as it may be, or as hard as it may be to resist, a good friendship is a horrible thing to lose.....GOOD LUCK!