So I'm in a bit of a dilemma. An "ex-love triangle"

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So I'm in a bit of a dilemma. An "ex-love triangle" if you will.
So in my grade 12 year of high school I became very close with my best friend and his girlfriend.
It got to the point where I was hanging out at their place a lot (yeah they lived together in hs).
Often our work schedules made it so my friend would be working and I'd be at their place alone with his gf.
I was attracted to her. But I respected their relationship, and my friendship with the guy.
My friend was very trusting. And him and I remain good friends to this day.
But they did break up over a year ago.
I've also stayed in touch with his girlfriend, and we've remained somewhat close as well.
One night I expressed my attraction to her, and asked her if she'd think it would be weird if we did stuff.
She kind of agreed, but I also mentioned how I thought maybe it wasn't such a good idea.
I told her that I didn't wan't to betray my friend. And we didn't do anything.
She kinda disagreed with me, and we kind of left it open.
But, as far as I know now; she's down to "do stuff", if I want.

Anyways. How do you think I should handle this?
Should I fuck my attractive female friend who was once my best friend's girlfriend?
If I do should I ever tell my friend? Should I ask him before what he would think if I did?
Should I just play it safe and not fuck her and remain friends with everyone?
I'd just hate for him to find out, and loose my friend over something so stupid.
But it's not often an attractive girl is wiling to have sex with me, so.
I just don't know what to do. Anything feedback will help.