I just would like to offer my 2 cents from a female perspectives view. First of all, true friends don't have anything negative to say about their "friends" to others, especially when they know it can stir the pot. I would highly consider re-evaluating this friendship. I would certainly not consider them one of my friends. Friends have each other's backs all the time.
Secondly, there are millions of people out there to date. In my opinion, your "so called friend" clearly disrespected you, not only by making you look bad, but then by his second choice to date someone, actually the exact person you have been involved with. We all know no one is perfect, but in a woman's world it is not ok to make a move on your friends ex, even if both parties have moved on. It creates nothing but issues, hard feelings, etc. (perhaps it is different in a man's world). It is a sign of respect, or disrespect in this case. Again, just my 2 pennies...and I am not expecting change back