From what I have gathered, this is your friend's ex-girlfriend? Forgive me if I'm wrong, but your first post is a little hard to follow.
Anyway, if he is your friend's ex, don't go for it. People will say its because of the guys code or whatever, but it just makes common sense. You start dating/fooling around with her, things get uncomfortable with you and her ex, she gets uncomfortable for making things uncomfortable, and it won't work out. I have "stolen" a couple of girls in the past, and it was never a good idea and never lasted long.
You also said she likes some other guy more than you? Again, don't go for it. You said she lost sleep last night for worrying if she made the right choice. IMO, when you are in a dilemma, its because you know you have to make the choice that will hurt someone, or make things worse, and you are trying to find a way out of it. She doesn't dig you like that man. You guys flirted a bit, made out, it's cool. But she's into this other guy, and like him more than you apparently. If she starts things up with out, she will always wonder what could have been with this other guy, and nothing will ever get serious with you. And then he will be the one she's sharing chocolate with (which is probably the best part about this story).
On the other hand, if this girl does like you, there is no harm in exploring it. As long as you don't get too attached while she still likes this other guy, go for it. Life is too short to quarrel over stupid things like this. Have fun with her (but be safem you never know...), and see what happens. The most important thing in all of this though is to remember not to get too serious unless you know this girl is really into you.