Not to get at your ex but you truly want to date their friend because you feel you would be better suited to date them as oppose to your ex.
Most Helpful Girl
As you said it yourself, once a relationship has ended you are free to do whatever. If you feel good with your ex's friend, then be with her. Maybe she is your "the one and only". You should care only about your own happiness.
And by the way, your exs friend should worry about this situation, not you, if she is going to be in a relationship with you.1