I'm surprised that nobody has pointed out that "this thread is worthless without pictures" yet.
You're making a very simple problem very complicated. How much is this friend worth to you looking forward? Be willing to accept that you may lose this friendship and that it may happen violently or just take a natural deteriorating collapse with the action you have in mind as the spark that started it all.
Just weight up what you have to gain and what you have to lose. As far as I can tell you don't have anything wonderful to gain. From the sound of it you just want to stick your penis in this particular woman to have done it. Nothing I read leads me to believe you have any ambitions of growing a relationship. So why her? Why not just hop in the sack with some total stranger? One night of sex, if that is what it is, can never compare to even a year of friendship. The fact that you are willing to risk a lifelong friend for this makes me think you either don't value that friend as much as you make it sound or you value this woman more than you are letting on.
Are you being superficial and only telling us about how sexy she is when really ther is much more to this?
I'm not gonna say "there are 1 million other women to choose from," but there's gotta be at least 20, so why her? But that's just me talking while everyone at Nike is encouraging you to Just Do It!(tm)
PS. And don't forget the pictures.
BTW, I should probably admit that I have slept with ex-partners of friends that I at one point never even considered looking at in that way. I like being loyal and an 'enforcer' of the will of my friends. So, it was with much regret that I had to hop in the sack with these ex-partners, on their request of course. It's back breaking work and it never ceases to amaze me how quick some people can 'turn'. Of course I was a complete bastard on some level, but to think that these couples that I otherwise had protected like my king and queen would dissolve, leaving me in the sack with one of them (no jokes please...) is actually gut wrenching. Too many drinks and a one-eyed friend with a knack for spellunking always helps to ensure that the situation repeats itself...again and again...
Which brings up a side point which I am not sure you have considered. I once cheated on a partner, something I never thought I would do. At first I was upset with myself because I had betrayed my girlfriend, but once the reality sank in it became apparent I had betrayed myself. It was a horrible feeling. I could not just tell someone "don't cry" or "it's not your fault, I'm an asshole and you can walk away from all this completely clean" and be done with it. Why? Because it was true. They were innocent and I was guilty. That may seem obvious and I may have been a self-centered prick for not having thought of that earlier, but it was a terrible revelation to have when you think you should be comforting someone you betrayed, but now you can't because "you are the disease".
