Firstly, she didn't keep her promise. Why? Maybe it's because to her she found a better guy than you. Secondly, you can't jump from being a lover to an instant friend. That is self-destruction.
You'll have to let her know currently you can't be friends. You need time to heal and recover from the breakup and move on. Until you've moved on, that's when you can be friends. You'll have to stop any communication with her till you've healed and recovered from the breakup.
You can't face her and her man and feel good about it. The breakup hasn't affected her. If it has, only a little bit by the fact she didn't end the relationship as you'd agreed.
There are many reasons why she wants the two of you to be friends but it will have to wait.