Well, if the girl you like has the same feelings for you, I think she should talk to your ex. What would you say to your ex? "Is it ok with you that I like your friend?" You don't need to explain or get her blessings for anything.
I think that the best thing to do would be to talk to your ex about this. Let her know that you respect her and you like her best friend. However, I hope that this was not the reason the two of you broke up.
She is Deffinately Off limits. She is your ex's best friend. If you have any decency please Do not go there. You are in danger of ruining a friendship. Us girls have an unwritten law. The bond of sisterhood comes before any man. You have no right to even consider going there in this lifetime. Your ex will despise you for it & it will look like you are being vindictive.