IR, only you know what the emotional issues are that effected your ex Kallah. You didn’t answer my inquiry if the Kallah had these issues before the engagement or if it is something that developed during the engagement. If these two girls are very close and the girl in question still supports the ex-kallah she will not do anything in the world to hurt her. In addition, there is a huge inyan of not hurting anyone when it comes to fertility and having children. Depending on how you broke it off with your ex-kallah and her family, would it be possible to approach them and say that obviously it was not meant to be, but Hashem does have a plan for all of us, and maybe his plan was not for the two of you, but for you to meet the friend and see her qualities. Let them know that you are not looking to hurt your former Kallah, but you would like her permission to date her friend because you do not want friction between the friends and do not want to break up a friendship. If it would cause her pain, then you should forget about it and move on.
Just her knowing that it might work out for her friend and you, she might love her friend enough to allow the two of you to try. She might not say yes right away, but giving her the opportunity to think about it, she might come around within weeks or a couple of months.
I am saying this because if you cause her additional pain knowing her emotional situation it can effect you and your future wife later on when you are trying to have children. That is a huge inyan. So maybe this is something you really should discuss with your Rav and review your whole situation in regards to the break up, how you handled it. How your ex feels about you right now. How she would feel if you dated her friend, etc. If she has no hard feelings towards you, maybe it is she that you should approach first.