Are there any exceptions?
Is nothing sacred or off limits in the 21st Century?
If she says its okay, does it make a difference?
These questions almost always come up in a girl's mind some time in their life time. Here is my own opinion on this subject;
If you are willing to date your friend's ex, then probably you are not really friends with this 'friend' of yours. Not only is it unethical, it's also kind of icky. Somethings are just common courtesy and a gesture of respect. No matter how much of a good guy you might think he is, no matter how much he might like you, it's just awkward, not to mention mean!
I have always told myself that I would never date one of my friend's exes, even if they dated for a day. If you make it to my friends/sisters list then your ex is automatically off limits. No matter who dumped who, it's a sensitive situation... and things will always be uncomfortable for the three parties involved.
Even if she gives you permission or she says she 'couldn't care less' or they broke up a long time ago? It will get sticky, she might not necessarily hate you or have anything against you but you will live with the guilt. I personally can't imagine my friend coming to say Congrats to me on my wedding with her ex. That is just not me, too awkward and definitely not worth the drama!