Friend or Back Stabber?

So you're in love... or at least you think you are, everything is great. He/she is funny, smart, attractive, caring, everything you want in a partner. You spend a lot of time together, you've his/her family, he/she has met yours. Life couldn't be better. Fast forward six months and everything changes, he/she doesn't want to be with you anymore. You find yourself lonely and heartbroken and just as you're trying to put the pieces back together, you find out your "best friend" is now dating him/her. So now not only are you heartbroken and lonely, you're angry and feeling betrayed. Even though you two have broken up, you still care for him/her, the wound is still fresh. Here's my question; would a TRUE friend date someone who hurt you so deeply after you cared so much about him/her? Is it ever okay to date your friend's ex? If so, in what situation would it be appropriate? Should someone feel betrayed if their friend dates an ex? Penny for your thoughts.