In my personal experience, which is nothing but observations, I would just do whatever is humanly possible to stay away from friends' ex-boyfriends.
No matter how long it's been, they always feel like he was once theirs and can therefore not be yours.
It might be hard for them to continue because their feelings of their ex moving on with his life is too much.
In the sense that somewhere deep down inside all womman, we want to believe that all out ex's were in some was fatally wounded by our leaving them.
We want to think that they still want us no matter what.
Or maybe it's just the hanging around him without being intimate that's hard for her.
Either way, the girl feels that somehow he's still her's and for him to go out with you is a betrayal of that.
Well there's my two cents on the matter.
Please ignore if I've gone and made you all the more confused.
Ta mate and good luck with this whole....thing.