I think an appropriate boundary would be - I will not be in a relationship with you while you are in one with another man.
Then stop meeting all her needs that you are providing her. Stop discussing things with her unless it is absolutely essential. She wants a divorce and you need to show her what a divorce is going to look like. Are you still going to be friends after the divorce and help her do things? Give her extra money and acts of service, gifts, words of affirmation?
