You have to determine if the agent's presence serves any purposes as witness or not. If she doesn't serve any purpose, juz tell her not to attend so that you can save the$200.
But, if you need them to attend the tribunal hearing officially as witness, it's then her duty as a citizen to attend in the position of a witness. Given this, you should apply to the court for an "order" to order the witness to attend. In this regard, they need to attend as an obligation to the Tribunal (in other words, the government) and you are not obliged to pay her any charge.