Talk to your friend about it, you might be suprised, he might take it pretty well. I had a friend come talk to me about hooking up with an ex I had recently broken up with once. And although I was kinda taken back by it, after talking about it and seeing he truly did care about her, I was cool with it. If you explain the attraction in a way thats not like your trying to jump in, perhaps he'll end the relationship and after a bit of time you and the girl will hook up.