What would a confident, trustworthy person do in this situation?
I think it depends on who you ask. In my opinion a confident and high value man with plenty of options would not allow a woman to treat him this way. If girl throws away a high value man and he has plenty of options then he wont waste his time waiting for that girl that just discarded him. He will think "if she isn't interested there are plenty of fish in the sea".
Most normal women will see this as fair however I believe borderlines see things different. Its a distorted way of thinking. They might believe they threw you away because they love you. They love you so much they couldn't be with you so they discarded you. Then when they see you with someone else they may feel like its a betrayal because they have feelings for you and your with someone else. Your not playing by their game. Even though they openly broke up with you their feelings are hurt and they may rage and punish you if you let them.
My opinion here is that if a truly confident and high value man meets this raging and punishing borderline he will ignore her threats and manipulation and simply tell her that she had her chance and blew it. If she threatens suicide then call the cops.
If you give in to the FOG she will try to apply then it will lead to a whole lot of pain.