There are so many other people in the world that you can date. It’s not the best idea to date the best friend of an ex, because there is too much drama attached to the idea.
The “good friend” has already told you that your relationship can go no further. She probably still feels some type of loyalty for her best friend, even if they aren’t as close as they once were. Her best friend, simply by nature of that type of relationship, probably heard all about you and probably gave advice to her friend when you and the ex were together. The “good friend” probably doesn’t want to breach any of that confidence and trust.
It doesn’t matter how or why you broke up with the ex, but if you date her best friend, that ex will probably be hurt and embarrassed and put into an awkward situation. Even if you and the “good friend” never cheated, the ex will now probably suspect that you did and feel even worse about the break up. Also, the ex will now have to hear all about you from her best friend (a horribly sucky situation) and the ex will probably have to be in situations where you are there with her best friend (another sucky situation) and the new girl will be forced to either keep her mouth shut or to avoid her best friend (a sucky betrayal either way you look at it).
After you broke up with the ex, she is probably trying to move on from you, but if you start dating her best friend, it’s like having all of that crap thrown into her face. Try to think about her feelings, even if she’s not your girlfriend anymore. If you start to date her best friend, your ex will probably lose her best friend too.
By getting involved with an exe’s best friend, you are creating a very unpleasnt threesome. Please try to find someone else to date.