This is a topic that may be viewed differently depending on whether you are male or female. Generally speaking, there is an unwritten code in the Girl's Friendship Handbook that you do not go after a friend's ex. At the very least, if you want to date your friend's ex you talk to them before making a move and if they are unhappy, don't go there.
I am speaking from personal experience. When someone you consider a friend makes a move on your ex after you have expressed that you are unhappy, it is hurtful. It's especially hurtful when they make promises they aren't going to keep about their intentions. That is when friendships get ruined.
If you do want to date your friend's ex, exercise caution. Check how your friend feels about it, be honest about what you want and maybe she will be okay with it. It is lies that are damaging.
Male perspectives on this topic are often varied. Some men would never date their friend's ex - I think there is a guy handbook too :p - but men are more logical thinking than women. Some men will think that as long as their friend's ex is available, then he is allowed to hit on her.
In reality, I believe the same rules apply whether you are male or female though. When persuing a friend's ex, tread with caution.
What are your thoughts on the subject?
You can read more on this topic in my article: Should You Date Your Friend's Ex?