My first question would be WHY did he break up with her? That is a very important question. I would definitely be careful not to get hurt the same way that she did. Obviously things were going well for a while if she is so hurt and can’t get over him, so what happened? That is important for you to know and for you to be objective about. Don’t bend over backwards in either direction not because you want to sympathize with your friend and call him a jerk, nor sympathize with him because you want to go out with him.
Then really think it through. What made him right for your friend to begin with and what made him wrong that they broke up? How alike are you and your friend and are you looking for the same thing? If this could really be someone that has the same values and lifestyle as you, discuss it with your friend and see what she says. A true friend who honestly loves you and believes he was a good man would tell you to go ahead, just like a sister would. A jealous friend who would say she would never forgive you is not a real friend who has YOUR best interests at heart. I would tell her that you are not looking to hurt her but that Hashem is in charge and that you will have to continue the conversation with your Rav.